REST: incorrect error while registration with empty username

Steps to reproduce:

1. Create POST request on registration with empty username, like this:

   <user xmlns="http://www.jtalks.org/namespaces/1.0">
     <username></username>
     <passwordHash>qwr</passwordHash>
     <email>bebebe666669912@mail.ru</email>
   </user>
   

Actual result:
<error code="user.username.already_exists"/>
Expected result:
<error code="user.username.length_constraint_violation"/>

Logs from catalina.out are attached.