Comment content without group provide message with not converted token

Problem/Motivation

When someoner comment content that is not in the group we receive an acivity with a message that contain not converted token of group title.

Steps to reproduce

- Create a content without group
- Add comment to it as another user
- Check activity srteam

Proposed resolution

Update the "isValidEntity" method in the "GroupActivityContext" to make shure it reurns true only if content have a group.