Add extra unittests for NullPropagation / ToString / AllowEqualsAndToStringMethodsOnObject is true

[StefH]

No description provided.

[Copilot]

Pull Request Overview

This PR adds extra unit tests to verify the behavior of null propagation and string conversion when AllowEqualsAndToStringMethodsOnObject is true. It also updates array initialization syntax in tests and refines a pattern match in the expression helper.

• Added two new unit tests for null propagation in QueryableTests.Where.cs.
• Updated array initialization formatting in ExpressionParserTests.cs.
• Replaced a negated type-check with a positive "is not" pattern in ExpressionHelper.cs.

Reviewed Changes

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

File	Description
test/System.Linq.Dynamic.Core.Tests/QueryableTests.Where.cs	Added two unit tests for null propagation and ToString conversion handling.
test/System.Linq.Dynamic.Core.Tests/Parser/ExpressionParserTests.cs	Reformatted array initialization using new C# syntax.
src/System.Linq.Dynamic.Core/Parser/ExpressionHelper.cs	Updated type-check to use the new pattern matching syntax.