[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.
.. _auth_module:
:mod:`cubicweb.pyramid.auth`
----------------------------
.. automodule:: cubicweb.pyramid.auth
.. autofunction:: includeme
.. autoclass:: UpdateLoginTimeAuthenticationPolicy
:show-inheritance:
:members: