fix: pass empty array to params parameter when [Arguments] has no values (#4572)

When [Arguments] is used with no arguments and the test method has a
params parameter, the parameter now correctly receives an empty array
instead of null. This matches standard C# behavior where calling a
method with params and no arguments passes an empty array.

Fixes #4561

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomhurst

TUnit.Core/Attributes/TestData/ArgumentsAttribute.cs

@@ -62,10 +62,14 @@ public sealed class ArgumentsAttribute : Attribute, IDataSourceAttribute, ITestR
 
     public ArgumentsAttribute(params object?[]? values)
     {
-        if (values == null || values.Length == 0)
+        if (values == null)
         {
             Values = [null];
         }
+        else if (values.Length == 0)
+        {
+            Values = [];
+        }
         else
         {
             Values = values;

TUnit.TestProject/ParamsArgumentsTests.cs

@@ -44,6 +44,17 @@ public async Task EmptyParamsArray(string name, params Type[] types)
         await Assert.That(types.Length).IsEqualTo(0);
     }
 
+    [Test]
+    [Arguments]
+    [Arguments("a")]
+    [Arguments("a", "b")]
+    [Arguments("a", "b", "c")]
+    public async Task ParamsOnlyWithEmptyArguments(params string[] args)
+    {
+        // When [Arguments] has no values, params should be an empty array, not null
+        await Assert.That(args).IsNotNull();
+    }
+
     [Test]
     [Arguments(1, "single")]
     public async Task SingleStringInParamsArray(int id, params string[] values)