Ensure env stanza is an empty dict if no seed value is given to papermill engine

[kevin-bates]

I'd like to reference our implementation of a Papermill Engine in the documentation for Gateway Provisioners to demonstrate how a user/developer can use Papermill against a Gateway server. When attempting to use the Papermill CLI to point at ElyraEngine, the notebook's execution would fail with the following stack (snippet):

  File "/opt/miniconda3/envs/elyra-dev/lib/python3.10/site-packages/jupyter_client/manager.py", line 397, in _async_start_kernel
    kernel_cmd, kw = await self._async_pre_start_kernel(**kw)
  File "/opt/miniconda3/envs/elyra-dev/lib/python3.10/site-packages/jupyter_client/manager.py", line 362, in _async_pre_start_kernel
    kw = await self.provisioner.pre_launch(**kw)
  File "/opt/miniconda3/envs/elyra-dev/lib/python3.10/site-packages/jupyter_client/provisioning/local_provisioner.py", line 200, in pre_launch
    return await super().pre_launch(cmd=kernel_cmd, **kwargs)
  File "/opt/miniconda3/envs/elyra-dev/lib/python3.10/site-packages/jupyter_client/provisioning/provisioner_base.py", line 157, in pre_launch
    env = kwargs.pop('env', os.environ).copy()
AttributeError: 'NoneType' object has no attribute 'copy'

because, by default, the env stanza would be None if using our engine directly.

This change enables the use of ElyraEngine as a referenceable example.

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[ptitzler]

LGTM!