Change default of AggregateUDFImpl::supports_null_handling_clause to false

[Jefffrey]

Which issue does this PR close?

• Part of Some aggregates silently ignore IGNORE NULLS and ORDER BY on arguments #9924

Rationale for this change

Setting a default of true is too permissive; we can see it allows specifying it on median for example even though that function doesn't take the config into account. It seems only array_agg, first_value and last_value actually respect the config this setting handles so it makes more sense to make this false by default.

What changes are included in this PR?

Change default of AggregateUDFImpl::supports_null_handling_clause to false (from true). Adjust array_agg, first_value and last_value to implement it as true.

Are these changes tested?

Existing tests, also added a negative case for median (expect SQL parsing to fail if providing null handling clause).

Are there any user-facing changes?

Behaviour change as default of a trait method is changing. Added section to upgrade guide.