patch 8.2.1491: Vim9: crash when compiling heredoc lines start with comment

Problem:    Vim9: crash when compiling heredoc lines start with comment.
Solution:   Skip over NULL pointers. Do not remove comment and empty lines
            when fetching function lines. (closes #6743)

src/autocmd.c

@@ -2310,7 +2310,11 @@ auto_next_pat(
  * Returns allocated string, or NULL for end of autocommands.
  */
     char_u *
-getnextac(int c UNUSED, void *cookie, int indent UNUSED, int do_concat UNUSED)
+getnextac(
+	int c UNUSED,
+	void *cookie,
+	int indent UNUSED,
+	getline_opt_T options UNUSED)
 {
     AutoPatCmd	    *acp = (AutoPatCmd *)cookie;
     char_u	    *retval;