[web/form] Relax condition for non-translating field names

we should test true condition instead of None, so if one raise a ValidationError
using an empty string as 'no-field' marker instead of None, we don't get the PO
file header that gettext likes to return when one attempt to translate an empty
string.

--HG--
branch : 3.25

cubicweb/web/views/formrenderers.py

@@ -172,7 +172,7 @@ class FormRenderer(AppObject):
                 else:
                     templstr = u' %s\n'
                 for field, err in errors:
-                    if field is None:
+                    if not field:
                         errormsg += templstr % err
                     else:
                         errormsg += templstr % '%s: %s' % (req._(field), err)