Fix Missing Link in Comment Notifications

[ayanaar]

Fixes #2538

This pull request introduces enhancements to the notification system, specifically addressing the issue reported in mozilla/pontoon#2538. It corrects the way notification verbs are displayed in scenarios where the associated strings have been deleted.

Changes:

• Implemented a check to accurately determine when a notification's target is missing (accounting for null or undefined values).
• Adjusted the notification verb phrasing to provide clearer context when the target string is absent, changing from "has added a comment in" to "has added a comment".

[mathjazz]

Well done!