Acceptance of optional attributes broken

When a form with optional attribute is submitted without this attribute being filled it is treated as attribute with 0-length value and Unity tries to assign such attribute with empty value when accepting the request.

Another related problem: mandatory attribute with schema set to minValues=0 is not accepted

Solution:

For all mandatory attributes: collect them applying schema rules (empty value = empty value, no values = no values, etc)

For all optional attributes: assume that attribute was not provided if no values were provided (UI allows for this if minValues =) or when all values provided are empty. If the attribute is not fulfilling those rules then assume it was provided and then apply the same verification rules as for mandatory attributes.