fix: handle empty candidate trait lists

Return empty JSON arrays from the API and normalize nullable values from older servers before rendering candidate match counts.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/App.test.tsx

@@ -69,7 +69,7 @@ function mockApi() {
             participant,
             score: 90,
             matchedTraits: ['Go'],
-            missingTraits: [],
+            missingTraits: null,
           },
         ])
       if (path === '/api/jobs/7/invitations' && method === 'GET') return response([])
@@ -129,6 +129,17 @@ describe('Go Guess frontend', () => {
     mockApi()
   })
 
+  it('renders candidate matches when an older API returns null trait arrays', async () => {
+    authenticate()
+    const user = userEvent.setup()
+    renderApp('/jobs/7')
+
+    await user.click(await screen.findByRole('tab', { name: /candidate match/i }))
+
+    expect(await screen.findByText('90%')).toBeInTheDocument()
+    expect(screen.getByText('Missing').nextElementSibling).toHaveTextContent('0')
+  })
+
   it('persists login, navigates from the login URL, and shows API jobs', async () => {
     const user = userEvent.setup()
     window.history.replaceState({}, '', '/login')

frontend/src/api.ts

@@ -115,7 +115,14 @@ export const api = {
       }),
     detachQuestion: (jobId: number, questionId: number) =>
       request<void>(`/jobs/${jobId}/questions/${questionId}`, { method: 'DELETE' }),
-    candidates: (jobId: number) => request<CandidateMatch[]>(`/jobs/${jobId}/candidates`),
+    candidates: async (jobId: number) => {
+      const matches = await request<CandidateMatch[]>(`/jobs/${jobId}/candidates`)
+      return matches.map((match) => ({
+        ...match,
+        matchedTraits: match.matchedTraits ?? [],
+        missingTraits: match.missingTraits ?? [],
+      }))
+    },
     invitations: (jobId: number) => request<Invitation[]>(`/jobs/${jobId}/invitations`),
     invitation: (jobId: number, invitationId: number) =>
       request<Interview>(`/jobs/${jobId}/invitations/${invitationId}`),