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>`

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nvim/regexp.c b/src/nvim/regexp.c
index b724439401..8241770bd3 100644
--- a/src/nvim/regexp.c
+++ b/src/nvim/regexp.c
@@ -4697,7 +4697,7 @@ regmatch (
      * If there is something on the regstack execute the code for the state.
      * If the state is popped then loop and use the older state.
      */
-    while (regstack.ga_len > 0 && status != RA_FAIL) {
+    while (!GA_EMPTY(&regstack) && status != RA_FAIL) {
       rp = (regitem_T *)((char *)regstack.ga_data + regstack.ga_len) - 1;
       switch (rp->rs_state) {
       case RS_NOPEN: