api.main: fix validation error for None response

FastAPI doesn't validate `None` when handler has specified
a response model. Fix the below validation error for GET node by ID
and GET group by ID requests:
```
{"detail":"1 validation error for UserGroup\nresponse\n  none
is not an allowed value (type=type_error.none.not_allowed)"}
```
The change has been effective from `fastapi 0.80.0` version.
Reference: tiangolo/fastapi#2725.

Signed-off-by: Jeny Sadadia <jeny.sadadia@collabora.com>

api/main.py

@@ -10,7 +10,7 @@
 
 import os
 import re
-from typing import List
+from typing import List, Union
 from fastapi import (
     Depends,
     FastAPI,
@@ -387,7 +387,7 @@ async def get_user_groups(request: Request):
     return paginated_resp
 
 
-@app.get('/group/{group_id}', response_model=UserGroup,
+@app.get('/group/{group_id}', response_model=Union[UserGroup, None],
          response_model_by_alias=False)
 async def get_group(group_id: str):
     """Get user group information from the provided group id"""
@@ -448,7 +448,7 @@ async def translate_null_query_params(query_params: dict):
     return translated
 
 
-@app.get('/node/{node_id}', response_model=Node,
+@app.get('/node/{node_id}', response_model=Union[Node, None],
          response_model_by_alias=False)
 async def get_node(node_id: str):
     """Get node information from the provided node id"""