Annotated tag is created even when message is empty

Summary

In /:namespace/:project_name/tags/new screen we say Optionally, enter a message to create an annotated tag.. This is not true, because we always create an annotated tag, even if message is empty

Steps to reproduce

1. Go to /:namespace/:project_name/tags/new screen
2. Fill in tag name
3. Leave message empty
4. Create tag
5. In your local repository: git pull to fetch the tag
6. git show replace-with-tag-name

It is an annotated tag with an empty message.

Possible fixes

I'm not sure if anybody actually minds that we always create an annotated tag. This also brings some consistency to the codebase as we do not have to support two code path. If this is true we should just update the label to prevent confusion.

/cc @DouweM @smcgivern @eReGeBe