fix(drizzle): coalesce jsonb_agg/json_group_array to empty array for hasMany select join-where

[GeorgS]

What?

Fix sub-folders not showing up in the folder browser (e.g. "Browse by folder" drawer) when those folders have no folderType set (universal folders).

Why?

Universal folders have zero rows in the payload_folders_folder_type join table. In drizzle/find/traverseFields.ts, the subquery that aggregates hasMany select field values uses jsonb_agg() (PostgreSQL) / json_group_array() (SQLite). When there are zero rows, these aggregates return NULL — not an empty array. The subsequent JSON path check NOT jsonb_path_exists(NULL, '$[*]') evaluates to NULL in SQL (not TRUE), so the WHERE clause silently excludes universal folders from the results.

SELECT NOT jsonb_path_exists(NULL, '$[*]');
-- Returns: NULL  -> row excluded from WHERE clause

SELECT NOT jsonb_path_exists(COALESCE(NULL, '[]'::jsonb), '$[*]');
-- Returns: true  -> row correctly included

This affects any query using exists: false (or any other operator) on a hasMany select field used inside a polymorphic join where clause.

How?

Wrap both aggregate subqueries in COALESCE so they always return an empty array instead of NULL when no rows exist:

- sql`(select jsonb_agg(t.value) from t where t.parent_id = parent.id)`
+ sql`coalesce((select jsonb_agg(t.value) from t where t.parent_id = parent.id), '[]'::jsonb)`

- sql`(select json_group_array(t.value) from t where t.parent_id = parent.id)`
+ sql`coalesce((select json_group_array(t.value) from t where t.parent_id = parent.id), '[]')`

This should also be backported to v3

Fixes #14821