Replace ga->ga_len > 0 checks with !GA_EMPTY(ga)

Used Coccinelle to perform the changes

```diff
@@
expression E;
@@

<...
(
- E.ga_len > 0
+ !GA_EMPTY(&E)
|
- E->ga_len > 0
+ !GA_EMPTY(E)
)
...>
```

`spatch --in-place --sp-file ga_empty.cocci <C_FILE>`

1 files changed, 3 insertions, 3 deletions

diff --git a/src/nvim/syntax.c b/src/nvim/syntax.c
index bfe13212ce..6fc62aed9a 100644
--- a/src/nvim/syntax.c
+++ b/src/nvim/syntax.c
@@ -951,7 +951,7 @@ static void syn_start_line(void)
    * Need to update the end of a start/skip/end that continues from the
    * previous line and regions that have "keepend".
    */
-  if (current_state.ga_len > 0) {
+  if (!GA_EMPTY(&current_state)) {
     syn_update_ends(TRUE);
     check_state_ends();
   }
@@ -2187,7 +2187,7 @@ syn_current_attr (
      */
     if (!syncing && !keep_state) {
       check_state_ends();
-      if (current_state.ga_len > 0
+      if (!GA_EMPTY(&current_state)
           && syn_getcurline()[current_col] != NUL) {
         ++current_col;
         check_state_ends();
@@ -2207,7 +2207,7 @@ syn_current_attr (
       && !(current_next_flags & (HL_SKIPNL | HL_SKIPEMPTY)))
     current_next_list = NULL;
 
-  if (zero_width_next_ga.ga_len > 0)
+  if (!GA_EMPTY(&zero_width_next_ga))
     ga_clear(&zero_width_next_ga);
 
   /* No longer need external matches.  But keep next_match_extmatch. */