Unexpected validation behaviour with empty strings in data#53

I've found some unexpected (to me) behaviour in WTForms-JSON where an empty string in an optional string field with a minimum length did not result in a validation error nor exclusion of the field from patch_data. I've written a small [test script](https://gist.github.com/kshade/a9a2e9ba514461bdbf02e17ce0ea4b1a), here is the output I'm getting from it: ``` WTForms-JSON version 0.3.3 json {} errors {} form.data {'name': None, 'potato': None} form.patch_data {} json {'name': ''} errors {} form.data {'name': None, 'potato': None} form.patch_data {'name': None} json {'name': 'a'} errors {'name': ['Field must be between 2 and 128 characters long.']} form.data {'name': 'a', 'potato': None} form.patch_data {'name': 'a'} json {'name': 'ab'} errors {} form.data {'name': 'ab', 'potato': None} form.patch_data {'name': 'ab'} json {'potato': None} errors {} form.data {'name': None, 'potato': None} form.patch_data {'potato': None} json {'potato': ''} errors {} form.data {'name': None, 'potato': None} form.patch_data {'potato': None} json {'potato': 'minimum'} errors {'potato': ['Invalid value, must be one of: maximum.']} form.data {'name': None, 'potato': 'minimum'} form.patch_data {'potato': 'minimum'} json {'potato': 'maximum'} errors {} form.data {'name': None, 'potato': 'maximum'} form.patch_data {'potato': 'maximum'} ``` Length validation doesn't happen with the second test case, but it also isn't removed from patch_data like in the first. Potato intuitively should only ever be "maximum" or not present, but both an empty string and None are accepted as None.