Fix bug in dropUndefinedKeys that kept references to original data

Author: Luca Forstner

packages/integration-tests/suites/public-api/startTransaction/circular_data/test.ts

@@ -13,7 +13,7 @@ sentryTest('should be able to handle circular data', async ({ getLocalTestPath,
 
   expect(eventData.contexts).toMatchObject({
     trace: {
-      data: { lays: { contains: { lays: { contains: '[Circular ~]' } } } },
+      data: { lays: { contains: '[Circular ~]' } },
     },
   });