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/path.c

@@ -893,7 +893,7 @@ expand_in_path (
   ga_init(&path_ga, (int)sizeof(char_u *), 1);
   expand_path_option(curdir, &path_ga);
   free(curdir);
-  if (path_ga.ga_len == 0)
+  if (GA_EMPTY(&path_ga))
     return 0;
 
   paths = ga_concat_strings(&path_ga);