Notification issues with the Follow tag feature

Problem/Motivation

1. User1 does not receive notification/email when user2 creates content with the tag that user1 followed.
2. The activity "Update node with following tag" created event if the node does not have tags.

Steps to reproduce

1.
- As user 1 create content with a tag
- As user 2 go to the content page and follow the tag
- As user 1 create another content with the same tag
- As user 2 you should receive notification about creating content with the tag you are following, but he/she didn't.

2.
- Update any topic/event without a tag
- Users will receive notification that followed tag was added to the content, but shouldn't

Proposed resolution

1. Change the logic for the activity contexts "follow_taxonomy_activity_context" and "follow_tag_activity_context".
2. Add additional condition in FollowTaxonomyActivityContext::isValidEntity to check if entity definitely have some tags.