Fix Get failing with a null exception

[RogerKratz]

Fixes #1920

[fredericDelaporte]

I do not see how we could "get" a collection without having it previously loaded as an uninitialized proxy, but I still have changed in the same way its loading logic. (The nearer to a "get" for a collection would be an entity load with a collection fetch, but that does not trigger batching.)

[RogerKratz]

I was about to suggest writing a test for "collection case", but reading your last comment I understand this change was just "a safety net", right?

[RogerKratz]

Now when always checking if keyIndex is null before calling GetSortedKeyIndexes, it's clearer to do a single null check inside that method I think.

[fredericDelaporte]

I was about to suggest writing a test for "collection case", but reading your last comment I understand this change was just "a safety net", right?

Yes. There is no ISession.Get equivalent for collections. But maybe, for example, some Merge cases could trigger the same issue. I do not know. Having a test case for collections would be better, if it is actually possible. But I am not sure it is.

[RogerKratz]

Great! Thanks!

[maca88]

@RogerKratz thanks for the provided test! It's my bad that I didn't tested this scenario.
@fredericDelaporte the provided fix is not correct as it can cause an endless loop where the entityKeys would be checked over and over again.
I've added additional checks in #1925 to prevent that and additional tests.
EDIT:
By changing only to a nullable parameter it seems that can also throw an IndexOutOfRangeException when batching is disabled.

[fredericDelaporte]

Better add them here I think. It would have avoided to lose the improvements already done here, like generating the async tests for GH1920 tests (and some white-space cleanup in them).

[maca88]

I've made a PR as I can't push directly to this one.

[fredericDelaporte]

Indeed, you are not among NHibernate "members".

[fredericDelaporte]

Replaced by #1925.