Make pg_get_statisticsobjdef_expressions return NULL
The usual behavior for functions in ruleutils.c is to return NULL when
the object does not exist. pg_get_statisticsobjdef_expressions raised an
error instead, so correct that.
Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/
20210505210947.GA27406%40telsasoft.com