fix(elections): return 200 with empty list when no elections exist

[HarshRajSinghania]

Fixes #189.

Summary

GET /api/election and GET /api/election?with_nominees=true returned HTTP 404 when the elections table was empty. An empty collection is a valid result, so the endpoint now returns 200 with [].

A 404 is still used for GET /api/election/{name} when a specific election slug does not exist.

Motivation

Issue #189: returning 404 when there are no rows is semantically incorrect and makes it look like the route or table is missing.

Implementation

• Removed the empty-list 404 checks in list_elections (src/elections/urls.py).
• Dropped the list-endpoint 404 OpenAPI response, since that status is no longer produced.
• Added unit tests that mock an empty result from the CRUD layer and assert status 200 and body [].

Testing

Unit tests added:

• tests/unit/test_elections.py::test__list_elections_returns_empty_list_when_none_exist
• tests/unit/test_elections.py::test__list_elections_with_nominees_returns_empty_list_when_none_exist

These tests call list_elections directly with mocked CRUD and do not require a database. Existing integration tests for non-empty lists were not changed.

I did not run the full integration suite here because it needs the project's Postgres test database.