API call /maintenance on POST is returning misleading response status

[ArturoQuintana]

• If the payload items field is empty, it is returning 201 when it should be 400.

Example:
# Sets up a wrong maintenance window data
payload = {
"description": "my MW on switch 2",
"start": start.strftime(TIME_FMT),
"end": end.strftime(TIME_FMT),
"items": []
}
reflected on end-to-end test: tests/test_e2e_15_maintenance.py::TestE2EMaintenance::test_024_create_mw_on_switch_should_fail_items_empty

In the case where the payload does not have an items field, it is returning 500 when should be 400.
Example:
# Sets up a wrong maintenance window data
payload = {
"description": "my MW on switch 2",
"start": start.strftime(TIME_FMT),
"end": end.strftime(TIME_FMT)
}
reflected on end-to-end test:
tests/test_e2e_15_maintenance.py::TestE2EMaintenance::test_026_create_mw_on_switch_should_fail_no_items_field_on_payload

• A not serializable JSON broke the application, and it should return a 415 error.

Example:
# Sets up a wrong maintenance window data
payload = {"a"}
reflected on end-to-end test:
tests/test_e2e_15_maintenance.py::TestE2EMaintenance::test_029_create_mw_on_switch_should_fail_wrong_payload