fix: pass empty array to params parameter when [Arguments] has no values

[thomhurst]

Summary

• Fixed bug where [Arguments] with no values passed null instead of an empty array to params parameters
• The ArgumentsAttribute constructor now distinguishes between empty arguments ([]) and explicit null ([null])
• Added test case to verify the fix

Fixes#4561

Test plan

• Verified new test ParamsOnlyWithEmptyArguments passes with 4 test cases
• Verified all 12 ParamsArgumentsTests pass
• Verified 105 source generator tests pass
• No snapshot test failures

🤖 Generated with Claude Code

[thomhurst]

Summary

Fixes bug where [Arguments] with no values passed null instead of an empty array to params parameters.

Critical Issues

None found ✅

Analysis

Changes Review:
The fix correctly distinguishes between three cases in the ArgumentsAttribute constructor (TUnit.Core/Attributes/TestData/ArgumentsAttribute.cs:63-73):

1. null passed → wraps as [null] (explicit null value)
2. Empty array [] → keeps as [] (empty params)
3. Non-empty array → uses values as-is

This aligns with standard C# behavior where calling a method with a params parameter and no arguments passes an empty array, not null.

Test Coverage:
The new test ParamsOnlyWithEmptyArguments (TUnit.TestProject/ParamsArgumentsTests.cs:47-58) validates the fix with 4 test cases covering the empty, single, double, and triple argument scenarios.

TUnit Rules Compliance:

• ✅ No dual-mode concerns - This is a runtime attribute change, not source generator output modification
• ✅ No snapshot test updates needed - No source generator output changed (verified PR description: "No snapshot test failures")
• ✅ No public API changes - Internal behavior fix only
• ✅ No reflection concerns - No new reflection usage
• ✅ No performance concerns - Single conditional check, minimal overhead
• ✅ No VSTest usage

Correctness:
The logic is sound. The original code treated both null and empty array identically (wrapping as [null]), which violated C# params semantics. The fix preserves the ability to pass an explicit null value via ArgumentsAttribute(null) while correctly handling the no-arguments case ArgumentsAttribute().

Verdict

✅ APPROVE - No critical issues