Fixes #12180 - lookup_value presence validation moved from lookup_key

Global parameters can have empty values so smart variables that are also global should be allowed to accept empty values too.
The validation on lookup_value to make sure it isn't empty is in the LookupKey class therefore affecting both VariableLookupKey and PuppetClassLookupKey.
This validation should affect on the child class - PuppetClassLookupKey.