add validation for mappings creation/edition

[syphax-bouazzouni]

Currently with the API we can create a mapping with empty classes, because no validation was done.

I implemented a size validator for the attribute classes to enforce the size 2.
https://github.com/ontoportal-lirmm/ontologies_linked_data/blob/development/lib/ontologies_linked_data/models/mappings/mapping.rb#L26

But this validator need to be enhanced to catch the sort of errors of empty string, e.g ["class_uri_1", ""] is valid currently and same for ["", ""].

Another enhancement is also to add the validator to goo like the existing ones.