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>

backend/internal/web/mapper/mapper_test.go

@@ -19,3 +19,10 @@ func TestInterviewOmitsReferenceAnswer(t *testing.T) {
 		t.Fatalf("expected participant-safe code question, got %#v", response)
 	}
 }
+
+func TestCandidateAlwaysUsesJSONArrays(t *testing.T) {
+	response := CandidateToWeb(servicemodel.CandidateMatch{})
+	if response.Matched == nil || response.Missing == nil {
+		t.Fatalf("candidate trait collections must not be nil: %#v", response)
+	}
+}