Empty class MethodDataSource fails construction in generated mode but supplies null in reflection mode #7442

Summary

An empty class-level [MethodDataSource] produces a failed test in source-generated mode but a passing test in reflection mode when the class constructor requires a string. Generated construction indexes the empty argument array; reflection supplies null for the missing required argument. The two discovery modes should apply the same missing-argument rule.

Reproduction

using TUnit.Core;

[MethodDataSource(nameof(Rows))]
public class EmptyConstructor(string value)
{
    public static System.Collections.Generic.IEnumerable<string> Rows() => [];

    [Test]
    public void Empty() { }
}

Run the application with --treenode-filter "/*/*/EmptyConstructor/*", then repeat with --reflection.

Expected and actual

Both modes retain the empty-source fallback test and report the missing required constructor argument consistently. Currently generated mode fails with IndexOutOfRangeException in its generated instance factory, while reflection mode passes after supplying null. This also affects method-inferred generic class fallback rows; a constructor that rejects null fails in both modes, with different errors.

Environment

• TUnit source commit 74a631d1fc637d5c06fa79454b32cf314df92a80, plus the scoped class-row filtering fix in fix(generator): infer generic class types from class MethodDataSource rows #7422. The non-generic reproduction does not use the changed generic inference path.
• .NET SDK 10.0.403, net10.0, macOS arm64.

Found while validating #7422. Please make missing required constructor arguments behave consistently in both execution modes.

Acceptance criteria

• Retain the empty-source fallback test and report a missing required constructor argument as a failure in both source-generated and reflection modes, rather than silently supplying null or exposing an array-index error.
• Add a regression for the non-generic reproduction above in both modes, asserting the fallback test count and missing-argument failure. Cover the method-inferred generic fallback case described above as well.
• Preserve valid supplied constructor arguments, including explicit null values where supported; this change concerns missing required arguments.
• Run affected generator snapshot tests and review intentional changes. Add engine integration coverage for CI; do not run TUnit.Engine.Tests locally. Verify Native AOT publishing if changing reflection paths.

The non-generic fix is independently actionable; #7422 is related context, not a prerequisite.