Query: Convert Entity comparison with null to comparison on key properties

[smitpatel]

Issue here:
We need to filter on qsre not null. But since it is not being materialized the value comes from the valuebuffer. ValueBuffer is tricky object here. When we use a composite shaper, the value buffer has values but we don't know corresponding value of particular qsre to compare it to null. And when it is valuebuffer for particular qsre only then _values becomes null instead of null value inside value buffer.

For SqlServer, which flattens query, we use composite shaper, we can translate such filters to Sql via entity equality rewriting.

For SQLite & InMemory where we generate different queries we need to guard against, _values in value buffer being null instead of actual value.

built on top of #6706

Resolves #6636

[smitpatel]

Updated with more nested tests.