Add spec for CompareService

The return result for the source or target branch being missing was
ambiguous before. It should be an empty compare to indicate no
comparison could be done.

spec/services/compare_service_spec.rb

@@ -19,5 +19,18 @@ describe CompareService do
 
       it { expect(subject.diffs.size).to eq(3) }
     end
+
+    context 'compare with target branch that does not exist' do
+      subject { service.execute(project, 'non-existent-ref') }
+
+      it { expect(subject).to be_nil }
+    end
+
+    context 'compare with source branch that does not exist' do
+      let(:service) { described_class.new(project, 'non-existent-branch') }
+      subject { service.execute(project, 'non-existent-ref') }
+
+      it { expect(subject).to be_nil }
+    end
   end
 end