[PATCH] Test coverage for pg_clear_attribute_stats() null arguments

Hi hackers,

pg_clear_attribute_stats() has no test coverage for its null-argument
checks, while pg_restore_relation_stats(), pg_restore_attribute_stats(),
pg_restore_ext_stats() and pg_clear_ext_stats() all do. It is called
exactly once in the whole test suite, and only on its success path.
That gap let a real bug live from v18 until 11ed011ae22([1])

The attached patch adds four cases, one per required argument. It is
test-only, with no behaviour change. The checks run at the top of the
function, before the relation lookup, so the expected output does not
depend on the state of the cluster.

[1]: https://github.com/postgres/postgres/commit/11ed011ae22a620b01a43634b5eea2f9af5c709c

Thanks,
Shihao