Change bms_add_range to be a no-op for empty ranges

In commit 84940644, bms_add_range was added with an API to fail with
an error if an empty range was specified.  This seems arbitrary and
unhelpful, so turn that case into a no-op instead.  Callers that require
further verification on the arguments or result can apply them by
themselves.

This fixes the bug that partition pruning throws an API error for a case
involving the default partition of a default partition, as in the
included test case.

Reported-by: Rajkumar Raghuwanshi <rajkumar.raghuwanshi@enterprisedb.com>
Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/16590.1532622503@sss.pgh.pa.us