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/undo.c

@@ -2419,7 +2419,7 @@ void ex_undolist(exarg_T *eap)
     }
   }
 
-  if (ga.ga_len == 0)
+  if (GA_EMPTY(&ga))
     MSG(_("Nothing to undo"));
   else {
     sort_strings((char_u **)ga.ga_data, ga.ga_len);