[web] Don't raise ProcessFormError if there is no value specified
a03376213747 introduced a slight modification in None / empty string values and
with it a regression where an exception is raised on empty values, while we
should simply consider no value is specified and return None.
.. _web_module:
:mod:`cubicweb.web`
===================
.. automodule:: cubicweb.web
Exceptions
----------
.. autoexception:: DirectResponse
.. autoexception:: InvalidSession
.. autoexception:: PublishException
.. autoexception:: LogOut
.. autoexception:: Redirect
.. autoexception:: StatusResponse
.. autoexception:: RequestError
.. autoexception:: NothingToEdit
.. autoexception:: ProcessFormError
.. autoexception:: NotFound
.. autoexception:: RemoteCallFailed