Fix: this prevents nginx to emergency shutdown if optional services are not started

[Neurone]

Description: This PR prevents nginx to emergency shutdown (host "xxx" not found in upstream) if an optional service like the mrv-sender, topic-viewer or mongodb-admin is not started (i.e., it's excluded from the docker-compose file);

Related issue(s):

Fixes # N/A

Notes for reviewer:

Checklist

• [x] Documented (Code comments, README, etc.)
• [x] Tested (unit, integration, etc.)