Support for None in mockgun when using order

[MHendricks]

When using mockgun if you try to use order and any of your results contain None in the ordered fields it will raise a error like this.

_mockgun.find("Shot", [], ["code"], order=[{"field_name": "description", "direction": "asc"}])

Traceback (most recent call last):
  File "C:\Program Files\Python39\lib\unittest\case.py", line 59, in testPartExecutor
    yield
  File "C:\Program Files\Python39\lib\unittest\case.py", line 592, in run
    self._callTestMethod(testMethod)
  File "C:\Program Files\Python39\lib\unittest\case.py", line 550, in _callTestMethod
    method()
  File "C:\blur\dev\shotgun_api3\tests\test_mockgun.py", line 476, in test_ordered_filter_operator
    shots = self._mockgun.find(
  File "C:\blur\dev\shotgun_api3\shotgun_api3\lib\mockgun\mockgun.py", line 308, in find
    results = sorted(results, key=lambda k: k[order_field], reverse=desc_order)
TypeError: '<' not supported between instances of 'NoneType' and 'str'

This pull request converts most of the data types to a empty value for the data type. I wasn't sure what to use for the bool and dict datatypes so it currently doesn't handle those.