Fix DetectChanges throwing when nullable nested complex property with nested collection is set to null

[Copilot]

Fixes #38632

Setting a nullable complex property to null during change detection throws InvalidOperationException: "must be initialized to a non-null value" when that property's type contains a collection with 2+ items, inside a ComplexCollection mapped with ToJson().

Root cause: InternalComplexCollectionEntry.GetEntry() checked whether the CLR collection was null before checking if a tracked entry already existed in _entries. During RemoveEntry's ordinal reindexing loop, the tracked entries are valid in-memory even though the parent CLR collection is null (because its parent complex property was just nulled out). With 1 item the reindex loop body never runs; with 2+ items it does, triggering GetEntry → null check → throw.

// Entity shape that triggers the bug:
entity.Items[0].Meta = null; // Meta.Entries had 2+ items
await context.SaveChangesAsync(); // throws before fix, saves correctly after

Changes:

• InternalEntryBase.InternalComplexCollectionEntry.cs — In GetEntry, move the _entries/_originalEntries lookup before the null CLR collection check. Existing tracked entries are returned directly, bypassing the (now-irrelevant) null check.
• ComplexCollectionJsonUpdateTestBase.cs — New model (EntityWithNullableMeta / ItemWithMeta / OptionalMeta / MetaEntry), seed data, and test Set_nullable_complex_property_with_nested_collection_to_null that exercises the 2-item path.
• ComplexCollectionJsonUpdateSqliteTest.cs / ComplexCollectionJsonUpdateSqlServerTest.cs — Provider-specific overrides with SQL assertions.

[Copilot]

Pull request overview

Fixes an InvalidOperationException thrown during change detection when a nullable nested complex property (containing a nested collection) is set to null inside a JSON-mapped complex collection (ToJson()), by ensuring existing tracked entries are consulted before validating CLR collection initialization.

Changes:

• Adjust InternalComplexCollectionEntry.GetEntry to return existing tracked entries before checking whether the underlying CLR collection is null, enabling ordinal reindexing during cleanup.
• Add a relational specification test model/seed + test coverage for the nullable complex property + nested collection scenario (2+ items path).
• Add provider-specific SQL assertions for SQLite and SQL Server.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/EFCore/ChangeTracking/Internal/InternalEntryBase.InternalComplexCollectionEntry.cs	Fixes GetEntry ordering to avoid throwing during cleanup/reindexing when parent CLR collection is null but tracked entries still exist.
test/EFCore.Relational.Specification.Tests/Update/ComplexCollectionJsonUpdateTestBase.cs	Adds new model/seed and a regression test covering nullable complex property nulled out with nested collection having 2+ elements.
test/EFCore.Sqlite.FunctionalTests/Update/ComplexCollectionJsonUpdateSqliteTest.cs	Adds SQLite SQL baseline assertion for the new regression test.
test/EFCore.SqlServer.FunctionalTests/Update/ComplexCollectionJsonUpdateSqlServerTest.cs	Adds SQL Server SQL baseline assertion for the new regression test.