When a new "Defect" Issue is created the Field FixedVersion cannot be required until it is determined if the bug report is valid and in what version it will be fixed. In this case we set the Field to optional in the "Field Configuration". When the Issue follows a workflow path where the Issue will be fixed then the FixedVersion must be always required. Now we can make the Field value required in the "Step Transition" Validator but when a user selects a "standard" JIRA action (i.e. Edit) then the FixedVersion field is optional again and can be set to "NUL". The next step validation will fail because the field value is not set.
Actually it makes more sense to associate the Conditions,Validators and Functions with the "Step" because many "Transitions" can lead to a step, but the "Step" occurs only once. "Transitions" configurations that lead to the same "Step" are redundent if the configuration is defined in the Step. The "Edit" and "Bulk Edit" actions would then also refer to the configuration of the "Step". This would also solve the problem applying "Conditions" to the "Create Issue" action.