gluon-web-model: fix issue with warning blocking save on private-wifi

The other bugfix which made this element inherit from AbstractValue
caused AbstractValue:validate() to be inherited aswell

Now added an if so validate only runs if a datatype is set
(since Element is meant as a generic way to extend web-model without
modifying web-model - also to add custom inputs - just hiding it behind an if sounds like a sane solution)