Fix selection of scoped labels

When sorting labels by user selection order, any missing labels
had index 0. If only single selected label was returned by UI,
then this label had index 0 too so it was not chosen as the "last
selected" label. To fix this, any missing labels have index -1.

ee/app/models/scoped_label_set.rb

@@ -28,7 +28,7 @@ class ScopedLabelSet
     by_index = label_ids_order.map.with_index { |id, idx| [id.to_i, idx] }.to_h
 
     label_ids.max do |id1, id2|
-      by_index[id1].to_i <=> by_index[id2].to_i
+      by_index.fetch(id1, -1) <=> by_index.fetch(id2, -1)
     end
   end
 

ee/spec/models/scoped_label_set_spec.rb

@@ -84,5 +84,11 @@ describe ScopedLabelSet do
 
       expect(set.last_id_by_order([kv_label1.id, kv_label3.id, kv_label2.id])).to eq(kv_label3.id)
     end
+
+    it 'returns last label present in the set ordered by custom order if there is single item' do
+      set = described_class.new('key', [kv_label1, kv_label3])
+
+      expect(set.last_id_by_order([kv_label3.id])).to eq(kv_label3.id)
+    end
   end
 end