Handle NULL fields in WRITE_INDEX_ARRAY

Handle NULL fields in WRITE_INDEX_ARRAY

Unlike existing WRITE_*_ARRAY macros, WRITE_INDEX_ARRAY needs to
handle the case that the field is NULL.  We already have the
convention to print NULL fields as "<>", so we do that here as well.
There is currently no corresponding read function for this, so reading
this back in is not implemented, but it could be if needed.

Reported-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/CAMbWs4-LN%3DbF8f9eU2R94dJtF54DfDvBq%2BovqHnOQqbinYDrUw%40mail.gmail.com