fix: handle empty search string in string_contains by checking search length only

Previously, the corner case for empty strings only returned true when both the source and search strings were empty. This fix changes the condition to return true whenever the search string is empty, as the empty string is always contained within any string.

src/wren_core.c

@@ -1156,8 +1156,8 @@ DEF_NATIVE(string_contains)
   ObjString* string = AS_STRING(args[0]);
   ObjString* search = AS_STRING(args[1]);
 
-  // Corner case, the empty string contains the empty string.
+  // Corner case, the empty string is always contained.
-  if (string->length == 0 && search->length == 0) RETURN_TRUE;
+  if (search->length == 0) RETURN_TRUE;
 
   RETURN_BOOL(wrenStringFind(vm, string, search) != string->length);
 }