fix: handle NULL values in /dags/{dag_id}/tasks order_by parameter

[Aliipou]

Closes #63927

Adds nullslast() handling to the SQLAlchemy order_by clause in the tasks listing endpoint to correctly handle NULL values and prevent incorrect sort ordering.

[boring-cyborg]

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[bugraoz93]

Thanks for the PR! I think changes should be on Pydantic model. Adding a validator is just for this cases and you can even manage time to execute like before after. There are different types of examples in our repo, could you please use that approach and add unit tests?
What I see in the issue is when wrong value passed the sorted won't work. That's why easier to do validations there