add Query.getSingleResultOrNull()

[gavinking]

Adds the new method I proposed in this discussion: https://github.com/jakartaee/persistence/issues/298#issuecomment-736682475.

[beikov]

Shouldn't it rather be something like this?

    default T getSingleResultOrNull() {
        try {
            return getSingleResult();
        } catch (NoResultException e) {
            return null;
        }
    }

Given that getResultStream was also added with a default implementation.

[gavinking]

Shouldn't it rather be something like this?

Sure, we could do that.

[mkalkowski83]

:+1:

[gavinking]

add to changelog

Done.

This is ready to merge.