Add WRITE_INDEX_ARRAY

Add WRITE_INDEX_ARRAY

We have a few WRITE_{name of type}_ARRAY macros, but the one case
using the Index type was hand-coded.  Wrap it into a macro as well.

This also changes the behavior slightly: Before, the field name was
skipped if the length was zero.  Now it prints the field name even in
that case.  This is more consistent with how other array fields are
handled.

Reviewed-by: Jacob Champion <pchampion@vmware.com>
Discussion: https://www.postgresql.org/message-id/c091e5cd-45f8-69ee-6a9b-de86912cc7e7@enterprisedb.com