Fix bug where labels would be assigned to issues that were moved

If you attempt to move an issue from one project to another and leave
labels blank, LabelsFinder would assign all labels in the new project
to that issue. The issue is that :title is passed along to the Finder,
but since it appears empty no filtering is done. As a result, all
labels in the group are returned. This fix handles that case.

Closes #23668

CHANGELOG.md

@@ -16,6 +16,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Refactor email, use setter method instead AR callbacks for email attribute (Semyon Pupkov)
 
 ## 8.13.1 (unreleased)
+  - Fix bug where labels would be assigned to issues that were moved
   - Fix error in generating labels
   - Fix reply-by-email not working due to queue name mismatch
   - Fixed hidden pipeline graph on commit and MR page !6895

app/finders/labels_finder.rb

@@ -35,6 +35,10 @@ class LabelsFinder < UnionFinder
   end
 
   def with_title(items)
+    # Match no labels if an empty title is supplied to avoid matching all
+    # labels (e.g. when an issue is moved)
+    return Label.none if params[:title] && params[:title].empty?
+
     items = items.where(title: title) if title
     items
   end

spec/finders/labels_finder_spec.rb

@@ -64,6 +64,12 @@ describe LabelsFinder do
 
         expect(finder.execute).to eq [group_label_2]
       end
+
+      it 'returns no labels if empty titles are supplied' do
+        finder = described_class.new(user, title: [])
+
+        expect(finder.execute).to be_empty
+      end
     end
   end
 end