CBMM attempts to send alerts to unconfigured Alertmanager

When running cbmultimanager by itself (local Docker instance) it attempts to reach out to Alertmanager, logging:

2022-01-18T10:50:50.244Z WARN (Alertmanager) Update error {"err": "failed to send alerts to : failed to request: Post \"/api/v2/alerts\": unsupported protocol scheme \"\""} 

Note how `am.BaseURL()` evaluates to `''` in the logged message.

This error should either be handled better, or CBMM shouldn't attempt the POST in the first place.