[FIX] import: import new records without xml_ids

This is related to revision 80b373f.

The above revision leads to the inability
to import new records without any `xml_id` set

Technically speaking, this is because
you cannot concatenate `False` with `'_'`, in
`xml_id + '_'`

opw-668962

openerp/addons/base/ir/ir_model.py

@@ -1044,6 +1044,7 @@ class ir_model_data(osv.osv):
         else:
             if mode=='init' or (mode=='update' and xml_id):
                 inherit_xml_ids = []
+                if xml_id:
                     for table, field_name in model_obj._inherits.items():
                         xml_ids = self.pool['ir.model.data'].search(cr, uid, [
                             ('module', '=', module),