Allow None in child_of operator

issue3190
review844002
(transplanted from 67208cffb07e8ab82e904a23f135fa8c6308d5a4)

--HG--
extra : transplant_source : g%20%8C%FF%B0%7E%8A%B8.%90J%23%F15%FA%8Cc%08%D5%A4