Blank screen if no mentions are available on the mentions tab (profile)

[ardacebi]

Bug Report

Current Behavior
If nobody mentioned you so far, the mentions tab page will be shown blank instead of a information text that warns the user that he/she hasn't been mentioned so far like the "no discussions on this category" warning.

Steps to Reproduce

1. Go to your profile
2. Click on mentions (nobody should mention you)
3. See error

Expected Behavior
The user should be presented with an information text like below.

---

Screenshots

Environment
Flarum Community
https://discuss.flarum.org

Possible Solution
Add an information message warning the user that he or she hasn't mentioned anywhere around the community.

[franzliedke]

Closing this as duplicate of #596