[FIX] fields property: do not create empty properties

fixes odoo#595
In the case where a property for the company exists but has no related record (e.g. in case of type m2o with no defined value), not setting a value to this field for a new record would create a new property (as browse_null is not an instance of browse_record)

openerp/osv/fields.py

@@ -1428,7 +1428,8 @@ def _fnct_write(self, obj, cr, uid, id, prop_name, id_val, obj_dest, context=Non
         default_val = self._get_default(obj, cr, uid, prop_name, context)
 
         property_create = False
-        if isinstance(default_val, openerp.osv.orm.browse_record):
+        if isinstance(default_val, (openerp.osv.orm.browse_record,
+                                    openerp.osv.orm.browse_null)):
             if default_val.id != id_val:
                 property_create = True
         elif default_val != id_val: