flaky systest `test_bucket_w_default_kms_key_name`

[cojenco]

Kokoro failed with flaky test:

______________________ test_bucket_w_default_kms_key_name ______________________

kms_bucket = <Bucket: gcp-systest-kms-1635439051141>
blobs_to_delete = [<Blob: gcp-systest-kms-1635439051141, default-kms-key-name, 1635439052948566>]
kms_key_name = 'projects/precise-truck-742/locations/us/keyRings/gcs-test/cryptoKeys/gcs-test'
alt_kms_key_name = 'projects/precise-truck-742/locations/us/keyRings/gcs-test/cryptoKeys/gcs-test-alternate'
file_data = {'big': {'hash': b'cEome4a+NYd7YIXzXQnR5Q==', 'path': '/tmpfs/src/github/python-storage/tests/data/five-point-one-mb-f...g'}, 'simple': {'hash': b'3Hkwjv2WvCnKjNR6Z3CboA==', 'path': '/tmpfs/src/github/python-storage/tests/data/simple.txt'}}

    @_helpers.retry_failures
    def test_bucket_w_default_kms_key_name(
        kms_bucket, blobs_to_delete, kms_key_name, alt_kms_key_name, file_data,
    ):
        blob_name = "default-kms-key-name"
        override_blob_name = "override-default-kms-key-name"
        alt_blob_name = "alt-default-kms-key-name"
        cleartext_blob_name = "cleartext"

        info = file_data["simple"]

        with open(info["path"], "rb") as file_obj:
            payload = file_obj.read()

        kms_bucket.default_kms_key_name = kms_key_name
        kms_bucket.patch()
        assert kms_bucket.default_kms_key_name == kms_key_name

        defaulted_blob = kms_bucket.blob(blob_name)
        defaulted_blob.upload_from_filename(info["path"])
        blobs_to_delete.append(defaulted_blob)

        assert defaulted_blob.download_as_bytes() == payload
        # We don't know the current version of the key.
>       assert defaulted_blob.kms_key_name.startswith(kms_key_name)
E       AttributeError: 'NoneType' object has no attribute 'startswith'

tests/system/test_kms_integration.py:152: AttributeError