fix: Getting list of tasks not working without name prefix

[athith-g]

Describe the bug
Making a GET request to the /tasks endpoint proTES with an empty body always returns an empty list of tasks, even when a task has been created. This is because of this line in task_runs.py. Without a name prefix, str(kwargs.get("name_prefix")) returns the string "None" meaning name_prefix is not None is always True. As a result, a filter of "^None" is applied when querying the db.

To Reproduce
Steps to reproduce the behavior:

1. Run proTES
2. Create a task (POST to /tasks with valid body (e.g. {"executors": [{"image": "alpine", "command": ["echo", "hello"]}]})
3. Get list of tasks (GET to /tasks with empty body)
4. Task list is empty

Expected behavior
The task list should contain the newly created task ID and status.

Screenshots
POST request made successfully:

GET request returns empty task list: