Using `$null` on non-terminal relationship returns empty results

[Helveg]

Since https://github.com/ppetzold/nestjs-paginate/pull/539 was merged, using filter.rel_A=$null while also specifying relations: ['rel_A.rel_B'] in the config results in an unexpected INNER JOIN on rel_A. This causes records where rel_A is NULL to be filtered out, leading to empty results.

Steps to Reproduce

1. Configure pagination with:

   {
     relations: ['rel_A.rel_B']
   }
   

2. Apply the filter:

   filter.rel_A.id=$null
   

3. The generated query INNER JOINs rel_A, excluding records where rel_A IS NULL.

Expected Behavior

• rel_A should be LEFT JOINED to preserve NULL values when filtering for primary keys of rel_A.id=$null.

Workaround

• Adding joinMethods: { rel_A: 'leftJoinAndSelect' } to the config prevents this issue.

  {
    relations: ['rel_A.rel_B'],
    joinMethods: { rel_A: 'leftJoinAndSelect' }
  }
  

• However, this behavior is unintended.

Suggested fix

Adding filters creates a map of columns to be left/inner joined here:

https://github.com/ppetzold/nestjs-paginate/blob/2b717aa85be7844980d526df908761cf189655e4/src/filter.ts#L394-L406

When a $null filter is added, then that column's value in the map should be set to leftJoinAndSelect, unless the user's configuration changes it.