fix: Allow reactions to be added to an existing comment without providing a `message`

[jwarby]

This allows users to add reactions to an existing comment without having to provide a message input. At the moment, trying to do this will fail because the API call to issues.updateComment fails if no body (message) param is provided.

This should allow things like this to work:

- uses: thollander/actions-comment-pull-request@v1
  with:
    comment_tag: foo
    reactions: eyes