fire.Fire(func) means that `func` cannot return an empty dict

[ubershmekel]

import fire

def x():
    return {}

fire.Fire(x)

Will cause this stack trace:

Traceback (most recent call last):
  File "E:/1stuff/Dropbox/dev/python/testfire/testing.py", line 7, in <module>
    z = fire.Fire(x)
  File "C:\Users\yuv\AppData\Local\Programs\Python\Python36-32\lib\site-packages\fire\core.py", line 145, in Fire
    _PrintResult(component_trace, verbose=component_trace.verbose)
  File "C:\Users\yuv\AppData\Local\Programs\Python\Python36-32\lib\site-packages\fire\core.py", line 181, in _PrintResult
    print(_DictAsString(result, verbose))
  File "C:\Users\yuv\AppData\Local\Programs\Python\Python36-32\lib\site-packages\fire\core.py", line 201, in _DictAsString
    len(str(key)) for key in result.keys()
ValueError: max() arg is an empty sequence

[guoruibiao]

python-fire is awesome cool.

[dbieber]

@ubershmekel Thanks for finding and fixing this bug! I'll respond to the pull request later in the week.