Update of the jQuery.wikibase.Claimview's isValid behavior

isValid() does not return true for an empty value now. Returning true in this case did result into
"save" buttons being enabled in some cases where the value was empty and where saving the value
would result in an error being thrown.

Change-Id: If4e1196551835776100382416430ae289f82c825
Merge "Update of the jQuery.wikibase.Claimview's isValid behavior"