**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/operators/test_dataflow.py
"""Test DataflowHook is created and the right args are passed to
        start_python_workflow.
        """
        start_python_hook = dataflow_mock.return_value.start_python_dataflow
        gcs_provide_file = gcs_hook.return_value.provide_file
        self.dataflow.execute(None)
        self.assertTrue(dataflow_mock.called)
        expected_options = {
            'project': 'test',
            'staging_location': 'gs://test/staging',
            'output': 'gs://test/output',
            'labels': {'foo': 'bar', 'airflow-version': TEST_VERSION}
        }
        gcs_provide_file.assert_called_once_with(object_url=PY_FILE)
        start_python_hook.assert_called_once_with(
            job_name=JOB_NAME,
            variables=expected_options,
            dataflow=mock.ANY,
            py_options=PY_OPTIONS,
            py_interpreter=PY_INTERPRETER,
            py_requirements=[],
            py_system_site_packages=False,
            on_new_job_id_callback=mock.ANY,
            project_id=None,
            location=TEST_LOCATION
        )
        self.assertTrue(self.dataflow.py_file.startswith('/tmp/dataflow'))
class TestDataflowJavaOperator(unittest.TestCase):