Don't fold a filtered subquery into a join whose key is an equality b…

[ilkertskn]

…etween nullable expressions on SQLite

SqliteSubqueryToJoinRewriter (#38692) rewrites JOIN (SELECT ... WHERE filter) ON key into JOIN table ON key AND filter. It runs at compile time, before SqlNullabilityProcessor, which deliberately leaves a join predicate consisting of a single top-level equality uncompensated because LINQ equijoins don't match null with null. After the fold the equality is nested under an AND, so it gets the regular a = b OR (a IS NULL AND b IS NULL) compensation and NULL keys start matching each other, returning extra rows compared to LINQ (and to SQL Server, which doesn't fold).

Skip the fold when the join key is a single equality whose two sides can both be null; when at least one side is non-nullable no compensation is generated anyway, so those joins keep being flattened. Adds inner and left join tests with a filtered inner source to NullSemanticsQueryTestBase.

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