**What happened**:

With the new Backport providers, when calling `DataflowCreatePythonJobOperator` with an empty `py_rerquirments` Dataflow commands always launch inside a virtualenv.

**What you expected to happen**:

The Dataflow command should run locally.


tests/providers/google/cloud/hooks/test_dataflow.py
mock_dataflow, mock_dataflowjob, mock_uuid):
        mock_uuid.return_value = MOCK_UUID
        mock_conn.return_value = None
        dataflow_instance = mock_dataflow.return_value
        dataflow_instance.wait_for_done.return_value = None
        dataflowjob_instance = mock_dataflowjob.return_value
        dataflowjob_instance.wait_for_done.return_value = None
        self.dataflow_hook.start_java_dataflow(  
            job_name=JOB_NAME, variables=DATAFLOW_VARIABLES_JAVA,
            jar=JAR_FILE)
        expected_cmd = ['java', '-jar', JAR_FILE,
                        '--region=us-central1',
                        '--runner=DataflowRunner', '--project=test',
                        '--stagingLocation=gs://test/staging',
                        '--labels={"foo":"bar"}',
                        '--jobName={}-{}'.format(JOB_NAME, MOCK_UUID)]
        self.assertListEqual(
            sorted(expected_cmd),
            sorted(mock_dataflow.call_args[1]["cmd"]),
        )
    @mock.patch(DATAFLOW_STRING.format('uuid.uuid4'))
    @mock.patch(DATAFLOW_STRING.format('_DataflowJobsController'))
    @mock.patch(DATAFLOW_STRING.format('_DataflowRunner'))
    @mock.patch(DATAFLOW_STRING.format('DataflowHook.get_conn'))
    def test_start_java_dataflow_with_multiple_values_in_variables(