Enable order with None in mockgun

[MHendricks]

Prevents an TypeError when checking if None > str

Prevents errors like this when using order on sg.find.

TypeError: '>' not supported between instances of 'NoneType' and 'str'

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)

Proposed Changes