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/ex_getln.c

@@ -4187,7 +4187,7 @@ static int ExpandRTDir(char_u *pat, int *num_file, char_u ***file, char *dirname
     }
     free(matches);
   }
-  if (ga.ga_len == 0)
+  if (GA_EMPTY(&ga))
     return FAIL;
 
   /* Sort and remove duplicates which can happen when specifying multiple