[release/10.0] Fix comparison to null for split entities

[roji]

Fixes #35293
Backports #37358

Description
When an entity uses entity splitting (mapped to multiple tables via SplitToTable), comparing it to null in a LINQ query throws InvalidOperationException: Sequence contains more than one element. The root cause is a call to SingleOrDefault() on GetViewOrTableMappings(), which returns multiple mappings for split entities. The fix replaces this with a switch expression that handles both single-table and multi-table (entity splitting) scenarios, selecting the principal table for split entities.

Customer impact
Users who use entity splitting and compare the split entity (or a navigation to it) to null in a LINQ query get an unhandled InvalidOperationException at query time. For example:

context.Set<InvoiceLine>().Select(x => new
{
    x.Id,
    Name = x.Invoice.Creator != null ? x.Invoice.Creator!.FullName : ""
});

A workaround exists: users can compare the key property instead of the entity (e.g. x.Invoice.CreatorId != null), but this is not immediately discoverable.

How found
User reported on EF Core 8.0.7 / 9.0.0. The issue has 2 upvotes and 3 additional comment authors asking for a fix or workaround, indicating multiple users are affected.

Regression
This is not a regression from a previous major version. Entity splitting was introduced in EF Core 7.0, and the bug has been present since then — the code always assumed a single table mapping when checking null comparisons.

Testing
1 new test added (Compare_split_entity_to_null) with SQL baseline verification on SQL Server.

Risk
Very low. The change is limited to a single method that resolves the table for null comparison checks. The fix adds handling for the multi-table case while preserving the existing single-table logic. Quirk added (Microsoft.EntityFrameworkCore.Issue35293) to allow opting out of the fix.

[Copilot]

Pull request overview

Backport addressing EF Core issue #35293 where comparing an entity-splitting entity to null in LINQ could throw due to assuming a single table/view mapping during null-equality rewrite.

Changes:

• Update null-comparison rewrite logic to handle multiple view/table mappings (entity splitting) and select the principal mapping, with an opt-out AppContext switch.
• Add a relational specification test covering split entity != null.
• Add SQL Server SQL baseline for the new test.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/EFCore.Relational/Query/RelationalSqlTranslatingExpressionVisitor.StructuralEquality.cs	Adjusts entity-null comparison rewrite to support split entities and adds an AppContext quirk switch.
test/EFCore.Relational.Specification.Tests/Query/EntitySplittingQueryTestBase.cs	Adds a new test covering comparing a split entity to null.
test/EFCore.SqlServer.FunctionalTests/Query/EntitySplittingQuerySqlServerTest.cs	Adds SQL Server AssertSql baseline for the new test.

[artl93]

Approved. Customer reported; blocking upgrade.