Remove redundant NULL check in clause_selectivity_ext() function

Hi everyone,

In the `clause_selectivity_ext()` function, there’s a comment regarding
a NULL clause check that asks, "can this still happen?". I decided to
investigate whether this scenario is still valid.

Here’s what I found after reviewing the relevant cases:

- approx_tuple_count(): The function iterates over a loop of other clauses.
- get_foreign_key_join_selectivity(): The function is invoked within an
`if (clause)` condition.
- consider_new_or_clause(): The clause is generated by
`make_restrictinfo()`, which never returns NULL.
- clauselist_selectivity_ext(): This function either checks
`list_length(clauses) == 1` before being called, or it is called within
a loop of clauses.

In other cases, the function is called recursively from
`clause_selectivity_ext()`.

If you are aware of any situations where a NULL clause could be passed
or if I’ve missed anything, please let me know. I’m also open to any
other suggestions.

--
Regards,
Ilia Evdokimov,
Tantor Labs LCC.