Fix Issue1244 test: Return Nothing in String-returning function → return null, not return default

String is a reference type so Nothing converts to null (not default).
Confirmed by ParameterTests.cs:35 which has the same pattern.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

Tests/CSharp/StatementTests/MethodStatementTests.cs

@@ -1587,7 +1587,7 @@ private static string Describe(int i)
                 return ""two"";
         }
 
-        return default;
+        return null;
     }
 }");
     }