[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.
.. -*- coding: utf-8 -*-
Writing text reports with RestructuredText
==========================================
|cubicweb| offers several text formats for the RichString type used in schemas,
including restructuredtext.
Three additional restructuredtext roles are defined by |cubicweb|:
.. autofunction:: cubicweb.ext.rest.eid_reference_role
.. autofunction:: cubicweb.ext.rest.rql_role
.. autofunction:: cubicweb.ext.rest.bookmark_role