[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.
#!/bin/sh -e
if [ "$1" = "purge" ] ; then
rm -rf /etc/cubicweb.d/
rm -rf /var/log/cubicweb/
rm -rf /var/lib/cubicweb/
fi
#DEBHELPER#
exit 0