patch 8.2.3456: Vim9: not all functions are tested with empty string argument

Problem:    Vim9: Not all functions are tested with an empty string argument.
Solution:   Add tests with empty strings. (Yegappan Lakshmanan, closes #8915)

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3456,
 /**/
     3455,
 /**/