Step6 crash if no inputs

Step6 will crash if the user does not add input files to their data directory before step2.
chris@chris-ubuntu20:/home/chris/Desktop/tests/step-7-test  (master)$ mic encapsulate step6
Downloading the base image and building your image
Adding inputs
Traceback (most recent call last):
  File "/usr/local/bin/mic", line 8, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.8/dist-packages/click-7.1.2-py3.8.egg/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/click-7.1.2-py3.8.egg/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.8/dist-packages/click-7.1.2-py3.8.egg/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.8/dist-packages/click-7.1.2-py3.8.egg/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.8/dist-packages/click-7.1.2-py3.8.egg/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.8/dist-packages/click-7.1.2-py3.8.egg/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/mic/__main__.py", line 370, in step6
    execute_using_docker(Path(mic_file))
  File "/usr/local/lib/python3.8/dist-packages/mic/component/executor.py", line 110, in execute_using_docker
    src_dir = create_execution_directory(mint_config_file, model_path)
  File "/usr/local/lib/python3.8/dist-packages/mic/component/executor.py", line 57, in create_execution_directory
    copy_inputs(mint_config_file, src_executions_dir)
  File "/usr/local/lib/python3.8/dist-packages/mic/component/executor.py", line 35, in copy_inputs
    for _, item in inputs.items():
AttributeError: 'NoneType' object has no attribute 'items'
mintproject / mic

Step6 crash if no inputs #139
