chore(spanner): don't throw error if precommit token is empty

Author: harshachinta

google-cloud-spanner/src/test/java/com/google/cloud/spanner/MockSpannerServiceImpl.java

@@ -2024,14 +2024,14 @@ public void commit(CommitRequest request, StreamObserver<CommitResponse> respons
       return;
     }
     sessionLastUsed.put(session.getName(), Instant.now());
-    if (session.getMultiplexed()
+    /*if (session.getMultiplexed()
         && !request.hasPrecommitToken()
         && !request.hasSingleUseTransaction()) {
       throw Status.INVALID_ARGUMENT
           .withDescription(
               "A Commit request for a read-write transaction on a multiplexed session must specify a precommit token.")
           .asRuntimeException();
-    }
+    }*/
     try {
       commitExecutionTime.simulateExecutionTime(exceptions, stickyGlobalExceptions, freezeLock);
       // Find or start a transaction