Fix comparison to null for split entities

[roji]

Fixes #35293

[Copilot]

Pull request overview

This PR fixes issue #35293 by adding support for null comparisons on split entities. Previously, the code failed when comparing a split entity to null because it didn't handle the multiple table mappings that result from entity splitting.

Key Changes

• Modified the null comparison logic to detect and handle entity splitting scenarios by selecting the principal table when multiple table mappings exist
• Added test coverage to verify that comparing split entities to null generates correct SQL with proper INNER JOINs

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/EFCore.Relational/Query/RelationalSqlTranslatingExpressionVisitor.StructuralEquality.cs	Replaced the single mapping lookup with a switch expression that handles both single mappings and entity splitting (multiple mappings) by selecting the principal table
test/EFCore.Relational.Specification.Tests/Query/EntitySplittingQueryTestBase.cs	Added base test that verifies split entity null comparisons work correctly with a simple entity splitting configuration
test/EFCore.SqlServer.FunctionalTests/Query/EntitySplittingQuerySqlServerTest.cs	Added SQL Server-specific test override that validates the generated SQL correctly performs INNER JOINs between main and split tables