[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.
.. _tools_module:
:mod:`cubicweb.pyramid.tools`
-----------------------------
.. automodule:: cubicweb.pyramid.tools
.. autofunction:: includeme
.. autofunction:: clone_user
.. autofunction:: cnx_attach_entity
.. autofunction:: cached_build_user
.. autofunction:: clear_cache