fix: TypeError in irc, mail, noff, rtd_hook plugin
    
    Fixes 'TypeError: not all arguments converted during string formatting' which was infact
    'sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed',
    caused by changes how way 'StringField' is handling the default value in 'wtforms' >= 3.0.0.
    In previous versions, 'None' was changed to an empty string, now it stays 'None'.
    Due to this change, tests and usage of IRC, Mail, Noff and rtd_hook Plugins failed,
    they contain 'nullable=False' fields without a default value in there Model definition.
    Model of the affected plugins adjusted, 'default=""' added to all 'nullable=False' fields.
    The behavior stays identical as with 'wtforms' < 3.0.0, which did exactly the same.
    
    References:
    https://github.com/wtforms/wtforms/commit/29c60458f3809bb43fb4007cfe710f3d7f4067d4
    https://github.com/wtforms/wtforms/issues/291