title: EasyAdmin - password update form gives error on empty password
body: <p>I'm using standard Symfony 4 User entity, created by make:auth and EasyAdmin 2.0.4.</p>
  
  <p>When editing existing User, password field is blank (which is good).</p>
  
  <p>On submitting existing User edit form, when leaving password field blank, I get following error:</p>
  
  <blockquote>
    <p>Expected argument of type "string", "NULL" given at property path "password".</p>
  </blockquote>
  
  <p>How to allow EasyAdmin to ignore password field on update, when its value is empty?</p>