Integration with H2

[DavideD]

Fix #929 It's a draft because I need to review it

[DavideD]

We had this error on CI (I need to have a look at it later):

Gradle Test Executor 1 > org.hibernate.reactive.CacheTest > testCacheWithNativeSQL FAILED
    java.util.concurrent.CompletionException: javax.persistence.PersistenceException: org.hibernate.HibernateException: java.util.concurrent.CompletionException: java.lang.NullPointerException
        at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
        at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319)
        at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:932)
        at java.base/java.util.concurrent.CompletableFuture.uniHandleStage(CompletableFuture.java:946)
        at java.base/java.util.concurrent.CompletableFuture.handle(CompletableFuture.java:2266)
        at java.base/java.util.concurrent.CompletableFuture.handle(CompletableFuture.java:143)
        at org.hibernate.reactive.session.impl.ReactiveSessionImpl.firePersist(ReactiveSessionImpl.java:678)
        at org.hibernate.reactive.session.impl.ReactiveSessionImpl.reactivePersist(ReactiveSessionImpl.java:662)
        at org.hibernate.reactive.util.impl.CompletionStages.lambda$loop$2(CompletionStages.java:177)
        at org.hibernate.reactive.util.impl.CompletionStages$ArrayLoop.next(CompletionStages.java:396)
        at org.hibernate.reactive.util.async.impl.AsyncTrampoline.lambda$asyncWhile$1(AsyncTrampoline.java:215)
        at org.hibernate.reactive.util.async.impl.AsyncTrampoline$TrampolineInternal.unroll(AsyncTrampoline.java:121)
        at org.hibernate.reactive.util.async.impl.AsyncTrampoline$TrampolineInternal.trampoline(AsyncTrampoline.java:102)
        at org.hibernate.reactive.util.async.impl.AsyncTrampoline.asyncWhile(AsyncTrampoline.java:197)
        at org.hibernate.reactive.util.async.impl.AsyncTrampoline.asyncWhile(AsyncTrampoline.java:215)
        at org.hibernate.reactive.util.impl.CompletionStages.loop(CompletionStages.java:356)
        at org.hibernate.reactive.util.impl.CompletionStages.loop(CompletionStages.java:340)
        at org.hibernate.reactive.util.impl.CompletionStages.loop(CompletionStages.java:177)
        at org.hibernate.reactive.util.impl.CompletionStages.applyToAll(CompletionStages.java:419)
        at org.hibernate.reactive.stage.impl.StageSessionImpl.persist(StageSessionImpl.java:137)
        at org.hibernate.reactive.CacheTest.lambda$testCacheWithNativeSQL$32(CacheTest.java:135)
        Caused by:
        javax.persistence.PersistenceException: org.hibernate.HibernateException: java.util.concurrent.CompletionException: java.lang.NullPointerException
            at app//org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:154)
            at app//org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:181)
            at app//org.hibernate.reactive.session.impl.ReactiveExceptionConverter.convert(ReactiveExceptionConverter.java:31)
            at app//org.hibernate.reactive.session.impl.ReactiveSessionImpl.lambda$firePersist$18(ReactiveSessionImpl.java:685)
            at [email protected]/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:930)
            ... 18 more
            Caused by:
            org.hibernate.HibernateException: java.util.concurrent.CompletionException: java.lang.NullPointerException
                at app//org.hibernate.reactive.session.impl.ReactiveExceptionConverter.convert(ReactiveExceptionConverter.java:31)
                ... 20 more
                Caused by:
                java.util.concurrent.CompletionException: java.lang.NullPointerException
                    at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
                    at java.base/java.util.concurrent.CompletableFuture.uniApplyNow(CompletableFuture.java:683)
                    at java.base/java.util.concurrent.CompletableFuture.uniApplyStage(CompletableFuture.java:658)
                    at java.base/java.util.concurrent.CompletableFuture.thenApply(CompletableFuture.java:2094)
                    at java.base/java.util.concurrent.CompletableFuture.thenApply(CompletableFuture.java:143)
                    at org.hibernate.reactive.id.impl.SequenceReactiveIdentifierGenerator.nextHiValue(SequenceReactiveIdentifierGenerator.java:45)
                    at org.hibernate.reactive.id.impl.BlockingIdentifierGenerator.generate(BlockingIdentifierGenerator.java:59)
                    at org.hibernate.reactive.id.impl.ReactiveGeneratorWrapper.generate(ReactiveGeneratorWrapper.java:38)
                    at org.hibernate.reactive.id.impl.IdentifierGeneration.generateId(IdentifierGeneration.java:140)
                    at org.hibernate.reactive.event.impl.AbstractReactiveSaveEventListener.reactiveSaveWithGeneratedId(AbstractReactiveSaveEventListener.java:122)
                    at org.hibernate.reactive.event.impl.DefaultReactivePersistEventListener.entityIsTransient(DefaultReactivePersistEventListener.java:179)
                    at org.hibernate.reactive.event.impl.DefaultReactivePersistEventListener.reactiveOnPersist(DefaultReactivePersistEventListener.java:128)
                    at org.hibernate.reactive.event.impl.DefaultReactivePersistEventListener.reactiveOnPersist(DefaultReactivePersistEventListener.java:58)
                    at org.hibernate.event.service.internal.EventListenerGroupImpl.lambda$fireEventOnEachListener$0(EventListenerGroupImpl.java:133)
                    at java.base/java.util.concurrent.CompletableFuture.uniComposeStage(CompletableFuture.java:1106)
                    at java.base/java.util.concurrent.CompletableFuture.thenCompose(CompletableFuture.java:2235)
                    at java.base/java.util.concurrent.CompletableFuture.thenCompose(CompletableFuture.java:143)
                    at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:133)
                    at org.hibernate.reactive.session.impl.ReactiveSessionImpl.firePersist(ReactiveSessionImpl.java:676)
                    ... 14 more
                    Caused by:
                    java.lang.NullPointerException
                        at java.base/java.util.concurrent.CompletableFuture.uniApplyNow(CompletableFuture.java:680)