[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.
:mod:`cubicweb.pyramid`
=======================
.. automodule:: cubicweb.pyramid
.. autofunction:: make_cubicweb_application
.. autofunction:: wsgi_application_from_cwconfig
.. autofunction:: wsgi_application
.. toctree::
:maxdepth: 1
:glob:
pyramid/*