patch 8.2.3335: Vim9: not enough tests run with Vim9

Problem:    Vim9: not enough tests run with Vim9.
Solution:   Run a few more tests in Vim9 script and :def function.  Fix that
            items(), keys() and values9) return zero for a NULL dict.
            Make join() return an empty string for a NULL list.  Make sort()
            return an empty list for a NULL list.

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3335,
 /**/
     3334,
 /**/