Increase test coverage for external triggers and RAVEN pipeline to 100%

Although currently the testing coverage for both the external trigger ingestion and the RAVEN alert pipeline are pretty good, work could be done to increase the coverage of all the involved tasks to 100%

See the report at the time of creating this issue: https://emfollow.docs.ligo.org/gwcelery/

|----------------------------------------|------------|---------|----------|----------|
| Module                                 | statements | missing | excluded | coverage |
|----------------------------------------|------------|---------|----------|----------|
| gwcelery/tasks/first2years_external.py | 48         | 1       | 0        | 98%      |
|----------------------------------------|------------|---------|----------|----------|
| gwcelery/tasks/external_skymaps.py     | 150        | 20      | 0        | 87%      |
|----------------------------------------|------------|---------|----------|----------|
| gwcelery/tasks/external_triggers.py    | 176        | 8       | 0        | 95%      |
|----------------------------------------|------------|---------|----------|----------|
| gwcelery/tasks/raven.py                | 140        | 7       | 0        | 95%      |
|----------------------------------------|------------|---------|----------|----------|