[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.

cubicweb-web: embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js