fix(completion): stop completion when completion is exhausted

Problem: Currently, completion is not stopped when completion is exhausted (no matching completion items), which also means `CompleteDone` is not called either. Furthermore, `CompleteDone` is called when pressing a special character (Backspace, Space, Esc) instead.

Solution: Correctly stop completion and trigger `CompleteDone` when completion is exhausted.

src/nvim/insexpand.c

@@ -1279,7 +1279,10 @@ void ins_compl_show_pum(void)
   }
 
   if (compl_match_array == NULL) {
-    if (compl_started && has_event(EVENT_COMPLETECHANGED)) {
+    // Stop completion when completion is exhausted
+    if (ctrl_x_mode_not_default()) {
+      ins_compl_prep(' ');
+    } else if (compl_started && has_event(EVENT_COMPLETECHANGED)) {
       trigger_complete_changed_event(cur);
     }
     return;