[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.
[theme]
inherit = pyramid
pygments_style = sphinx.pygments_styles.PyramidStyle
stylesheet = cubicweb.css
[options]
logo = logo-cubicweb.svg
favicon = cubicweb.ico
in_progress = false
outdated = false
canonical_url =