I noticed that validations on a field require an entry (although not defined as a mandatory field). If the field is empty, an error occurs. But it may be empty. Is there a function here so that the validation is only executed if the field is not empty?
Yes, what you suggest is very reasonable: if a filed isn't required, and a user left it empty, then the filed should be valid, even if there are validation constraints defined on the field. We have an RFE for this, with more information, and I've added a +1 from you.