fix: correct test expectation for first aggregate with include_nil? (#683)

The test was asserting buggy behavior. With `sort(title: :asc_nils_first)`
and `include_nil?: true`, the `first` aggregate should return `nil` when the
first sorted record has a nil value, not skip to the next non-nil value.

Author: sevenseacat

test/aggregate_test.exs

@@ -874,7 +874,9 @@ defmodule AshSql.AggregateTest do
       |> Ash.Changeset.manage_relationship(:post, post, type: :append_and_remove)
       |> Ash.create!()
 
-      assert "match" ==
+      # With sort(title: :asc_nils_first), nil comes first.
+      # With include_nil?: true, we should get nil (not skip to the next non-nil value)
+      assert nil ==
                Post
                |> Ash.Query.filter(id == ^post.id)
                |> Ash.Query.load(:first_comment_nils_first_include_nil)