Handle JSON null token when reading collections mapped to JSON

[Copilot]

A JSON column/property mapped to a primitive collection (e.g. List<string>?) threw InvalidOperationException: Invalid token type: 'Null' when the stored value was a JSON null literal instead of an array.

The three collection reader/writers only accepted a StartArray token at the top level. Nulls are usually short-circuited before reaching the reader (e.g. FromJsonPropertyString), but the value-converter path (CollectionToJsonStringConverter.FromJsonString) and nested JSON documents can drive the reader directly to a Null token.

Changes

• JsonValueReaderWriter.FromJsonString: short-circuits on a top-level JsonTokenType.Null and returns null — one central place covers all reader/writers without needing per-class handling. Return type updated from object to object? to accurately reflect that null is a valid return value.
• EFCore.baseline.json: updated FromJsonString signature to object?.
• Tests: added Can_read_write_null_value_of_collection_of_string_JSON_values, Can_read_write_null_value_of_collection_of_int_JSON_values, and Can_read_write_null_value_of_collection_of_nullable_int_JSON_values to JsonTypesTestBase, asserting that a JSON "null" value round-trips as a null collection while normal arrays still read correctly. The Can_read_and_write_JSON_value helper now also exercises the FromJsonString("null") path for null values.

public object? FromJsonString(string json, object? existingObject = null)
{
    ...
    return readerManager.CurrentReader.TokenType == JsonTokenType.Null
        ? null
        : FromJson(ref readerManager, existingObject);
}

[Copilot]

Pull request overview

This PR fixes JSON primitive-collection reader/writers so that a top-level JSON null token materializes as a null collection rather than throwing, aligning behavior across direct-reader usage (e.g., value converters) and nested JSON reads.

Changes:

• Updated JsonCollectionOfReferencesReaderWriter and JsonCollectionOfStructsReaderWriter to return null when the current token is JsonTokenType.Null.
• Updated JsonCollectionOfNullableStructsReaderWriter to return null when the current token is JsonTokenType.Null (after the existing None advance).
• Added JsonCollectionReaderWriterTest to assert "null" round-trips to null across reference, struct, and nullable-struct collection reader/writers.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/EFCore/Storage/Json/JsonCollectionOfReferencesReaderWriter.cs	Accept top-level Null token and return null rather than throwing.
src/EFCore/Storage/Json/JsonCollectionOfStructsReaderWriter.cs	Accept top-level Null token and return null rather than throwing.
src/EFCore/Storage/Json/JsonCollectionOfNullableStructsReaderWriter.cs	Accept top-level Null token and return null (with existing None handling).
test/EFCore.Tests/Storage/Json/JsonCollectionReaderWriterTest.cs	New unit tests validating "null" reads as null for various collection shapes.

[AndriySvyryd]

Superseded by #38498