wtforms-json: How can I not allow None/empty string values?

Question

I'm building a simple JSON API using Flask and wtforms-json. I have a problem where fields are allowed to be None, even when they really should not be, for example something this:

wtforms.BooleanField('state', validators=[validators.Optional(), validators.AnyOf([True, False])])

still lets through explicit None (or null) values somehow instead of failing validation or removing the field from the form's patch_data() alltogether (although None sometimes should be valid). Same with String fields, setting a default value doesn't help either.

I've written a small test script to illustrate the problem. Setting name to '' results in it being None in the patch data instead of being missing like the second accepted variable.

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'}