Omit WHERE clause for NOT labels filter when empty

This clause will always be true when the labels provided are
non-existent. In certain cases, having this SQL clause results in
PG picking a slow query plan.

Changelog: performance

app/finders/issuables/label_filter.rb

@@ -82,6 +82,8 @@ module Issuables
       if root_namespace
         label_ids = find_label_ids(label_names).flatten(1)
 
+        return issuables if label_ids.empty?
+
         issuables.where(label_link_query(target_model, label_ids: label_ids).arel.exists.not)
       else
         issuables.where(label_link_query(target_model, label_names: label_names).arel.exists.not)