Fixed edge case in engagement score normalisation

[CallmeMehdi]

Changes proposed ✍️

• Added handling of edge case for engagement normalization when the tenant doesn't have any activities

• Screenshots (front-end changes only)

    - ...  
  

Checklist ✅

• Label appropriately with type:feature 🚀, type:enhancement ✨, type:bug 🐞, or type:documentation 📜.
• Tests are passing.
• New backend functionality has been unit-tested.
• Environment variables have been updated
  • Front-end: frontend/.env.dist
  • Backend: backend/.env.dist, backend/.env.dist.staging, backend/.env.dist.staging.
  • Configuration docs have been updated.
  • Team members only: update environment variables in Password manager and update the team
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.
• All changes have been tested in a staging site.
• All changes are working locally running crowd.dev's Docker local environment.

[joanreyero]

Can we test in staging before merging?