Adapt pg_upgrade test to pg_lsn output format difference

Adapt pg_upgrade test to pg_lsn output format difference

Commit 2633dae2e487 added some zero padding to various LSNs output
routines so that the low word is always 8 hex digits long, for easy
human consumption.  This included the pg_lsn datatype, which breaks the
pg_upgrade test when it compares the pg_dump output of an older version.
Silence this problem by setting the pg_lsn columns to NULL before the
upgrade.

Discussion: https://postgr.es/m/202507071504.xm2r26u7lmzr@alvherre.pgsql