title: is it possible to store a PhoneNumberType from SQLalchemy as null?
body: <p>I'm creating an API with webargs and python, and I have a condition that a user must supply either a phone number or email address. If the user provides an email address only, then when sqlalchemy tries to instantiate a PhoneNumberType, I get the expected error:</p>
  
  <p>sqlalchemy_utils.types.phone_number.PhoneNumberParseException: (1) (1) The phone number supplied was None.</p>
  
  <p>my question is how can i make this database field able to accept a null phone number and just keep that entry null?</p>
  
  <p>Here is part of my user model:</p>
  
  <pre><code>import flask_sqlalchemy as sa
  ...
  phone         = db.Column(db.Unicode(20), nullable=True)
  country_code  = db.Column(db.Unicode(8), nullable=False)
  _phone        = sa.orm.composite(PhoneNumber, phone, country_code)
  </code></pre>
  
  <p>country code is needed for other reason, but is there a way to make say... _Phone null ??</p>