No notifications with the default graph (or in triple mode)

Describe the bug There are no notifications when the subscription (and update) queries use the default graph (by not specifying any graph in queries).

To Reproduce Steps to reproduce the behavior via the SEPA Playground:

1. Add subscription:

SELECT ?o WHERE {
  <http://sepatest/testsubj> <http://sepatest/hasValue> ?o
}

2. Issue query:

DELETE {
    <http://sepatest/testsubj> <http://sepatest/hasValue> ?o
} INSERT {
    <http://sepatest/testsubj> <http://sepatest/hasValue> "c"
} WHERE {
  OPTIONAL {
    <http://sepatest/testsubj> <http://sepatest/hasValue> ?o
}}

3. There are no notifications in the Subscribe tab. However, the same query as was used to subscribe, returns the new value.

Expected behavior I expect to see notification about new value of the subject in the Subscribe tab of the Playground.

System information(please complete the following information):

• OS: [Windows 10]
• Engine version [SEPA Broker Ver 0.9.11] (though the jar file i downloaded is named engine-0.9.12.jar)
• SparqlEnpoint used [blazegraph 2.1.6-SNAPSHOT]

Additional context Yes, the simple fix from my side would be to use some explicitly specified graph, however, it turns out to be impossible, because I am planning to use Blazegraph with inference enabled as the endpoint, however, Blazegraph doesn't support inference in quads mode, and it obviously doesn't support graphs in triples mode.

Hi! thank you for using SEPA and posting such a great issue description. We were able to reproduce the bug and we are working on solving it. We'll keep you posted 👍🏻

Dear @m-hatter thank you very much! We fixed the issue and pushed new version of dev branch. Please check it and let us know!