[web] fix muledit rendering bug form.cw_col might be None and explicitly passing None to the selector chain may crash. Most __call__ methods of selectors are defined as in : def __call__(self, cls, req, rset, row=None, col=0, **kwargs) and then manipulate col as if it's an integer.

# cube's specific schema
"""

:organization: Logilab
:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""