[Firebase] Warnings in ConformanceTest.php

[bshaffer]

There are two warnings in the Firebase Conformance Tests that we should look into (since we just released a new major version):

PHP Warning:  Undefined array key "fieldPaths" in /google-cloud-php/google-cloud-php/Firestore/tests/Unit/ConformanceTest.php on line 193
PHP Warning:  foreach() argument must be of type array|object, null given in /google-cloud-php/Firestore/tests/Unit/ConformanceTest.php on line 193