chore: make precommit token check emulator-proof

Author: olavloite

.github/workflows/integration-tests-against-emulator.yaml

@@ -10,7 +10,7 @@ jobs:
 
     services:
       emulator:
-        image: gcr.io/cloud-spanner-emulator/emulator:1.5.37
+        image: gcr.io/cloud-spanner-emulator/emulator
         ports:
           - 9010:9010
           - 9020:9020

google/cloud/spanner_v1/transaction.py

@@ -328,7 +328,7 @@ def before_next_retry(nth_retry, delay_in_seconds):
             # successfully commit, and must be retried with the new precommit token.
             # The mutations should not be included in the new request, and no further
             # retries or exception handling should be performed.
-            if commit_response_pb.precommit_token:
+            if commit_response_pb._pb.HasField("precommit_token"):
                 add_span_event(span, commit_retry_event_name)
                 commit_response_pb = api.commit(
                     request=CommitRequest(