**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.


airflow/providers/google/cloud/hooks/dataflow.py
return ['--labels={}={}'.format(key, value)
                    for key, value in labels_dict.items()]
        if py_requirements is not None:
            with TemporaryDirectory(prefix='dataflow-venv') as tmp_dir:
                py_interpreter = prepare_virtualenv(
                    venv_directory=tmp_dir,
                    python_bin=py_interpreter,
                    system_site_packages=py_system_site_packages,
                    requirements=py_requirements,
                )
                command_prefix = [py_interpreter] + py_options + [dataflow]
                self._start_dataflow(
                    variables=variables,
                    name=name,
                    command_prefix=command_prefix,
                    label_formatter=label_formatter,