Fix null complex property values in SaveChanges for table splitting

[AndriySvyryd]

Add complex property builder overloads for nullable structs

Part of #31376

cc @artl93

[Copilot]

Pull Request Overview

This PR fixes an issue with null complex property values during SaveChanges when using table splitting by adding support for nullable struct complex properties. The main purpose is to properly handle nullable value types in complex property configurations and update tracking.

Key changes:

• Add complex property builder overloads that accept nullable struct expressions (Expression<Func<TEntity, TProperty?>>)
• Update complex type resolution to unwrap nullable types correctly
• Fix column modification logic to handle null complex properties in table splitting scenarios

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/EFCore/Metadata/Internal/InternalTypeBaseBuilder.cs	Updates complex type resolution to unwrap nullable types using UnwrapNullableType()
src/EFCore/Metadata/Builders/EntityTypeBuilder*.cs	Adds overloaded methods for nullable struct complex properties and collections
src/EFCore/Metadata/Builders/ComplexPropertyBuilder*.cs	Adds overloaded methods for nullable struct complex properties within complex types
src/EFCore/Metadata/Builders/ComplexCollectionBuilder*.cs	Adds overloaded methods for nullable struct complex collections within complex types
src/EFCore.Relational/Update/ColumnModification.cs	Updates GetCurrentValue to handle null complex properties in table splitting
test/EFCore.Specification.Tests/ComplexTypesTrackingTestBase.cs	Updates tests to use nullable License properties and adds new test cases
Various test files	Refactors test inheritance to use relational base classes and removes nullable disable pragmas

Comments suppressed due to low confidence (1)

[cincuranet]

FYI, the failures are from InMemory.