fix(test): correct edit-metadata assertion from toBeUndefined to toBeNull

The inspectMetadata function returns null (not undefined) when no EXIF
data is present. Fix the test assertion to match.

tests/integration/api.test.ts

@@ -3763,7 +3763,7 @@ describe("Edit metadata", () => {
       });
       expect(res.statusCode).toBe(200);
       const body = JSON.parse(res.body);
-      expect(body.exif).toBeUndefined();
+      expect(body.exif).toBeNull();
     });
 
     it("rejects request with no file", async () => {