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

Used Coccinelle to perform the changes

@@
expression E;
@@

<...
(
// E.ga_len == 0 is isomorphic to !E.ga_len
- E.ga_len == 0
+ GA_EMPTY(&E)
|
- E->ga_len == 0
+ GA_EMPTY(E)
)
...>

src/nvim/regexp.c

@@ -5021,7 +5021,7 @@ regmatch (
     /*
      * If the regstack is empty or something failed we are done.
      */
-    if (regstack.ga_len == 0 || status == RA_FAIL) {
+    if (GA_EMPTY(&regstack) || status == RA_FAIL) {
       if (scan == NULL) {
         /*
          * We get here only if there's trouble -- normally "case END" is