fix: exclude nils from unsorted list aggregates

[wtsnz]

The problem

On AshPostgres, a list aggregate without a sort includes nil values, although include_nil? defaults to false:

# Comments titled "bbb", nil and "aaa".
Post |> Ash.Query.aggregate(:titles, :list, :comments, field: :title) |> Ash.read_one!()
# => titles: ["bbb", nil, "aaa"]  (expected no nil)

A sorted list, or one over a relationship with a sort, correctly leaves the nil out. This predates the aggregate strategies refactor.

Why

In the lateral strategy, the sorted branch of the list aggregate handles include_nil?. It either adds FILTER (WHERE field IS NOT NULL), or folds the nil check into the aggregate's own filter. The unsorted branch builds a plain array_agg(field) (or array_agg(DISTINCT field)), with no nil handling.

The fix

The unsorted branch now handles nils the same way as the sorted one:

• include_nil?: true keeps the plain array_agg.
• An aggregate with its own filter hands the field to filter_field/6, which adds not is_nil(field) to that filter.
• Otherwise it adds FILTER (WHERE field IS NOT NULL), with or without DISTINCT.

Tests

The regression test is in AshPostgres, where the bug shows up: wtsnz/ash_postgres#6. It covers an unsorted list, uniq?, a list with its own filter, and include_nil?: true.

• It fails on the published AshSQL 0.7.6, returning [nil, "aaa", "aaa", "bbb"].
• It passes with this branch (ASH_SQL_VERSION=local).

Also checked:

• AshSQL's own tests pass (20).
• AshPostgres's full suite has the same 39 failures as main in the local test database, and nothing new.

Found by

The data-layer conformance suite (wtsnz/ash#7), gap unsorted-list-nil. With this branch, both Postgres scenarios behind it (values.list_unsorted, root.list_unsorted) pass, and no other result changes.