Add test case for empty local_nb_nodes.json file

[alyssadai]

If a user tries to deploy the f-API with an empty local_nb_nodes.json file, the following error is thrown:

ERROR:    Traceback (most recent call last):
  File "D:\neurobagel\envs\fed_api_venv\Lib\site-packages\starlette\routing.py", line 677, in lifespan
    async with self.lifespan_context(app) as maybe_state:
  File "C:\Users\adai\AppData\Local\Programs\Python\Python311\Lib\contextlib.py", line 204, in __aenter__
    return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File "D:\neurobagel\federation-api\app\main.py", line 22, in lifespan
    await util.create_federation_node_index()
  File "D:\neurobagel\federation-api\app\api\utility.py", line 113, in create_federation_node_index
    local_nodes = parse_nodes_as_dict(LOCAL_NODE_INDEX_PATH)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\neurobagel\federation-api\app\api\utility.py", line 96, in parse_nodes_as_dict
    if valid_nodes:
       ^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'valid_nodes' where it is not associated with a value

We should handle this kind of case and also add a test case for it.

[surchs]

🚀 Issue was released in v0.1.0 🚀