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)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 612d478..4a6da4d 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -612,7 +612,7 @@
     int
 do_cmdline(
     char_u	*cmdline,
-    char_u	*(*fgetline)(int, void *, int, int),
+    char_u	*(*fgetline)(int, void *, int, getline_opt_T),
     void	*cookie,		// argument for fgetline()
     int		flags)
 {
@@ -638,7 +638,7 @@
     msglist_T	*private_msg_list;
 
     // "fgetline" and "cookie" passed to do_one_cmd()
-    char_u	*(*cmd_getline)(int, void *, int, int);
+    char_u	*(*cmd_getline)(int, void *, int, getline_opt_T);
     void	*cmd_cookie;
     struct loop_cookie cmd_loop_cookie;
     void	*real_cookie;
@@ -1482,9 +1482,9 @@
  */
     int
 getline_equal(
-    char_u	*(*fgetline)(int, void *, int, int),
+    char_u	*(*fgetline)(int, void *, int, getline_opt_T),
     void	*cookie UNUSED,		// argument for fgetline()
-    char_u	*(*func)(int, void *, int, int))
+    char_u	*(*func)(int, void *, int, getline_opt_T))
 {
 #ifdef FEAT_EVAL
     char_u		*(*gp)(int, void *, int, int);
@@ -1512,7 +1512,7 @@
  */
     void *
 getline_cookie(
-    char_u	*(*fgetline)(int, void *, int, int) UNUSED,
+    char_u	*(*fgetline)(int, void *, int, getline_opt_T) UNUSED,
     void	*cookie)		// argument for fgetline()
 {
 #ifdef FEAT_EVAL
@@ -1541,7 +1541,7 @@
  */
     char_u *
 getline_peek(
-    char_u	*(*fgetline)(int, void *, int, int) UNUSED,
+    char_u	*(*fgetline)(int, void *, int, getline_opt_T) UNUSED,
     void	*cookie)		// argument for fgetline()
 {
     char_u		*(*gp)(int, void *, int, int);