Backport last commit of CMS7-5832 Custom Field Validation Service to 2.22.xx branch

In CMS trunk, in a final commit of CMS7-5832 a patch by Michael Aartun was applied in JcrFieldValidator: removing an 'else' before checking validator for null.

It was never backported to 2.22.xx branch.
I will commit the easy-fix and assign to Frank for review.