Avoid different-size pointer-to-integer cast

Avoid different-size pointer-to-integer cast

Buildfarm member mamba is unhappy that I wrote "(Datum) NULL" in commit
28d534e2ae0a:
  https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mamba&dt=2026-04-07%2005%3A08%3A08
Use "(Datum) 0" which is what we do everywhere else.

Discussion: https://postgr.es/m/CANWCAZaOs_+WPH13ow33Q==+FwBwVZkqzm4vND=WEB4_NBmv1Q@mail.gmail.com