title: Symfony3 Form component trying to pass null to a type hinted method in PHP 7
body: <p>In my entity class I have defined all the expected argument types for the setters and return types of the getters.
  Later, when I have a form which uses the said class, I get an error if some of the fields in the form is empty because the form component tries to pass null to the setter instead of string.</p>
  <p>I get the following exception when I submit the form:</p>
  <blockquote>
  <h3>Expected argument of type &quot;string&quot;, &quot;NULL&quot; given</h3>
  <p><sup>500 Internal Server Error - InvalidArgumentException</sup></p>
  </blockquote>
  <p>The exception is thrown from <code>vendor/symfony/symfony/src/Symfony/Component/PropertyAccess/PropertyAccessor.php at line 254</code></p>
  <p>Is there a way to convert the &quot;null&quot; value to empty string before passing it to the object, and let the validator argue about it?</p>