copy_data is not a supported option with this sub-command of ALTER
SUBSCRIPTION, which would not make a variable related to it initialized
after parsing the option set in DefElems.  A refresh could then refer to
it.

Author: Ranier Vilela
Reviewed-by: Peter Smith
Discussion: https://postgr.es/m/CAEudQAp5P8nr=ze2Gv=BMj=DJFZnrvendZCZcC-fos3QiDe2sg@mail.gmail.com