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

@@ -1253,7 +1253,7 @@ static int ins_compl_build_pum(void)
 /// Also adjusts "compl_shown_match" to an entry that is actually displayed.
 void ins_compl_show_pum(void)
 {
-  if (!pum_wanted() || !pum_enough_matches()) {
+  if (!pum_wanted()) {
     return;
   }
 
@@ -1278,10 +1278,15 @@ void ins_compl_show_pum(void)
     }
   }
 
-  if (compl_match_array == NULL) {
+  if (compl_match_array == NULL || !pum_enough_matches()) {
     if (compl_started && has_event(EVENT_COMPLETECHANGED)) {
       trigger_complete_changed_event(cur);
     }
+  }
+
+  // Stop completion when completion is exhausted
+  if (compl_match_array == NULL && ctrl_x_mode_not_default()) {
+    ins_compl_prep(' ');
     return;
   }