pg_stat_progress_cluster: do not default to CLUSTER

Hello

Since ac58465e061, pg_stat_progress_cluster is not built on
pg_stat_get_progress_info() but on pg_stat_progress_repack, and maps
the new REPACK command name back onto the two names the view used to
report. The mapping is a CASE ending in an unconditional ELSE
'CLUSTER'.

pg_stat_get_progress_info() hands pid and datid to every caller, but
NULLs other parameters if the caller no has privileges of the role
running the command.

That includes both command and repack_index_relid used in that CASE,
and we can display a VACUUM (FULL) as CLUSTER because of this default
branch.

The attached patch fixes this to report NULL instead. I also attached
a (LLM generated) script that reproduces the issue.