htmlform: Handle null from HTMLFormField::getDefault in multiselects
HTMLFormField::getDefault is documented to return mixed, which includes
null, but WebRequest::getText is documented to allow strings only.
The use of WebRequest::getText cannot be replaced with
WebRequest::getVal in the multiselects, because there are working with
\n and WebRequest::getText provides normalisation for \r\n
(cherry picked from commit c7b84d9e579a8d2fc42ed6e4a8e43ceaaaf387e1)