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>`

src/nvim/path.c

@@ -1094,7 +1094,7 @@ gen_expand_wildcards (
       free(t);
     }
 
-    if (did_expand_in_path && ga.ga_len > 0 && (flags & EW_PATH))
+    if (did_expand_in_path && !GA_EMPTY(&ga) && (flags & EW_PATH))
       uniquefy_paths(&ga, p);
     if (p != pat[i])
       free(p);