User-defined formatter validate functions have to handle None.

[dpopowich]

When defining custom formatters with SwaggerFormat, the validate function has to handle None values. So all my validators are littered with code like this:

def my_validator(val):
   if val is not None:
      # real validation here

Shouldn't None be special-cased, left to x-nullable checking, never calling the validator when value==None is True? This is quite tedious.

[adamhadani]

+1 Might be related to my PR #143