Fix rendering assignee changed comments without assignee (#27927)

Fix rendering assignee changed comments without assignee (#27927)

When an assignee changed event comment is rendered, most of it is
guarded behind the assignee ID not being 0. However, if it is 0, that
results in quite broken rendering for that comment and the next one.
This can happen, for example, when repository data imported from outside
of Gitea is incomplete.

This PR makes sure comments with an assignee ID of 0 are not rendered at
all.

---

Screenshot before:
<img width="272" alt="Bildschirm­foto 2023-11-05 um 20 12 18"
src="https://github.com/go-gitea/gitea/assets/42910/7d629d76-fee4-4fe5-9e3a-bf524050cead">
The comments in this screenshot are:
1. A regular text comment
2. A user being unassigned
3. A user being assigned
4. The title of the PR being changed

Comments 2 and 3 are rendered without any text, which indents the next
comment and does not leave enough vertical space.

Co-authored-by: Giteabot <teabot@gitea.io>