Fix shellcheck error SC2155

Close https://github.com/neovim/neovim/pull/11765

scripts/vim-patch.sh

@@ -597,9 +597,10 @@ list_missing_previous_vimpatches_for_patch() {
   set +u  # Avoid "unbound variable" with bash < 4.4 below.
   if [[ -z "${missing_list[*]}" ]]; then
     msg_ok 'no missing previous Vim patches'
+    set -u
     return 0
   fi
-  fi
+  set -u
 
   local -a missing_unique
   local stat