Untyped [Arguments(null)] method row on a generic class silently produces no test #7141

Summary

An untyped [Arguments(null)] method row on a generic test class, with nothing else to infer the class's type argument, produces zero tests in both source-gen and reflection modes. There is no diagnostic or failed test, so the test silently disappears.

public class NullRowOnGenericClass<T>
{
    [Test]
    [Arguments(null)]
    public void Run(T value) { }
}

Where

• Source gen: InferClassTypesFromMethodArguments in src/TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs returns null for a null params array.
• Reflection: InferClassTypeArgumentsFromMethodRow in src/TUnit.Engine/Discovery/ReflectionTestDataCollector.cs returns null for ArgumentsAttribute.IsNullValuesArray.

#7106 (PR #7111) chose this deliberately so that [Arguments(null)] does not break a where T : struct constraint (fixture NullArrayMethodRowOnConstrainedClass). The problem is that when no other row or [GenerateGenericTest] provides an instantiation, the row is dropped without any signal.

Reproduce

Add the class above to TUnit.TestProject, then run --treenode-filter "/*/<ns>/NullRowOnGenericClass/*" in either mode. Discovery finds 0 tests.

Expected

Either infer object, matching how a generic method treats [Arguments(null)] (#7120), when that satisfies the constraints, or report a clear diagnostic or failed test saying T cannot be inferred. Do the same in both modes.

Found while fixing #7120.