Fix NullReferenceException when accessing null complex properties in TPH with shared columns

[Copilot]

Fixes #37335

Problem

When two derived types in TPH share a nullable complex property mapped to the same column, accessing the property when null throws NullReferenceException, particularly during materialization or when calling entry.OriginalValues.ToObject().

Changes

Modified PropertyAccessorsFactory.CreateMemberAccess

Previously disabled null checks unconditionally when fromDeclaringType was true. Now preserves null checks for nullable reference type complex properties:

• Checks if the property is a complex property (not a regular scalar property)
• Checks if both the declaring entity and complex property types are reference types (not structs)
• Respects the incoming addNullCheck parameter

Added test coverage in ComplexTypesTrackingSqlServerTest

• Integrated tests into the existing ComplexTypesTrackingSqlServerTest class using transactions for database isolation
• Can_read_original_values_with_TPH_shared_complex_property_column_null() validates materialization and original values access with null complex properties
• Can_read_original_values_with_TPH_shared_complex_property_column_with_value() validates the same with populated complex properties

Related Discussion

This PR replaces #[original-pr-number] and now targets main instead of release/10.0 as requested by @AndriySvyryd.

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[AndriySvyryd]

@copilot Rebase onto main

[Copilot]

@copilot Rebase onto main

I've successfully rebased the branch onto main locally (commits are now based on 84b8d15). However, I cannot force push the rebased history due to authentication constraints with the available tools. The rebase is complete and ready - a maintainer with push access will need to force push or I can create a new PR with the rebased commits if preferred.

[AndriySvyryd]

@copilot Create a new PR targeting main

[Copilot]

@copilot Create a new PR targeting main

I've created a new branch copilot/fix-nullreferenceexception-mapping-main that targets main (based on commit 84b8d15). All 5 commits have been cherry-picked onto main:

• a92da60a59: Add test to reproduce TPH complex property sharing issue
• c9bad85f66: Fix NullReferenceException when accessing null complex properties in TPH
• 5551e86b88: Refine fix to only add null checks for nullable reference complex properties
• 84a810cfe3: Remove redundant assignment and improve code clarity
• 2f91a34c3c: Move tests to ComplexTypesTrackingSqlServerTest and use transactions

However, I cannot push this new branch due to authentication constraints. A maintainer will need to:

1. Push the local branch copilot/fix-nullreferenceexception-mapping-main to origin, OR
2. Let me know if there's another approach to create the PR targeting main

[AndriySvyryd]

Superseded by #37695