HTTP GET on non-existing proposal returns empty content

Summary

Using the REST API to get a non-existing proposal through /api/v3/proposals/{pid} returns with HTTP status code 200 and empty content.

From an interactive Python session:

>>> res = client._session.get(f'{client.baseurl}/proposals/does_not_exist')
>>> res.url
'https://scicat-backend-dev.desy.de/api/v3/proposals/does_not_exist'
>>> res.status_code
200
>>> res.content
b''

As a workaround, I am checking for empty content to determine, if the proposal already exists. Using the HTTP status code would be preferred.

Steps to Reproduce

• Authenticate as proposalIngestor through the API
• Call /api/v3/proposals/{pid}, where {pid} refers to a non-existing proposal
• Receive HTTP status code 200 and empty response

Current Behaviour

API call returns HTTP status code 200 and empty content in the response

Expected Behaviour

API should return HTTP status code 404 not found.

Start at the REST endpoint handling GET /api/v3/proposals/{pid} and reproduce the request for a non-existing proposal as proposalIngestor. Trace how the empty response is produced and verify that the missing proposal returns HTTP 404 rather than 200 with empty content. Done means the expected status is returned consistently, with a regression test if the project has endpoint tests.