Remove some unnecessary null checks (#1801)

### Motivation and Context

The culture will never be null here so these fallbacks aren't ever used.

### Contribution Checklist
<!-- Before submitting this PR, please make sure: -->
- [x] The code builds clean without any errors or warnings
- [x] The PR follows SK Contribution Guidelines
(https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
- [x] The code follows the .NET coding conventions
(https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions)
verified with `dotnet format`
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

dotnet/src/SemanticKernel/SkillDefinition/SKFunction.cs

@@ -922,7 +922,7 @@ private static void TrackUniqueParameterType(ref bool hasParameterType, MethodIn
                     // If that fails, try with the invariant culture and allow any exception to propagate.
                     try
                     {
-                        return converter.ConvertFromString(context: null, cultureInfo ?? CultureInfo.CurrentCulture, input);
+                        return converter.ConvertFromString(context: null, cultureInfo, input);
                     }
                     catch (Exception e) when (!e.IsCriticalException() && cultureInfo != CultureInfo.InvariantCulture)
                     {
@@ -974,7 +974,7 @@ private static void TrackUniqueParameterType(ref bool hasParameterType, MethodIn
                         return null!;
                     }
 
-                    return converter.ConvertToString(context: null, cultureInfo ?? CultureInfo.InvariantCulture, input);
+                    return converter.ConvertToString(context: null, cultureInfo, input);
                 };
             }