Fix issue with loading tags on the compare page

Not all repositories have tags, so we need to deefault to an empty array
to not fail when we filter on tags. To be sure, we also default to an
empty array for branches.

app/assets/javascripts/projects/compare/components/revision_dropdown_legacy.vue

@@ -65,8 +65,8 @@ export default {
       return axios
         .get(endpoint)
         .then(({ data }) => {
-          this.branches = data.Branches;
-          this.tags = data.Tags;
+          this.branches = data.Branches || [];
+          this.tags = data.Tags || [];
         })
         .catch(() => {
           createFlash({

changelogs/unreleased/322724-fix-compare-page-dropdown-loading.yml

+---
+title: Fix issue with loading the repository compare page
+merge_request: 55058
+author:
+type: fixed

spec/frontend/projects/compare/components/revision_dropdown_legacy_spec.js

@@ -62,6 +62,20 @@ describe('RevisionDropdown component', () => {
     expect(wrapper.vm.tags).toEqual(Tags);
   });
 
+  it('sets branches and tags to be an empty array when no tags or branches are given', async () => {
+    axiosMock.onGet(defaultProps.refsProjectPath).replyOnce(200, {
+      Branches: undefined,
+      Tags: undefined,
+    });
+
+    createComponent();
+
+    await axios.waitForAll();
+
+    expect(wrapper.vm.branches).toEqual([]);
+    expect(wrapper.vm.tags).toEqual([]);
+  });
+
   it('shows flash message on error', async () => {
     axiosMock.onGet('some/invalid/path').replyOnce(404);