Fix rendering assignee changed comments without assignee (#27927) (#27952)

Backport #27927 by @invliD

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: Sebastian Brückner <code@nik.dev>

templates/repo/issue/view_content/comments.tmpl

@@ -4,7 +4,8 @@
 		{{$createdStr:= TimeSinceUnix .CreatedUnix ctx.Locale}}
 
 		<!-- 0 = COMMENT, 1 = REOPEN, 2 = CLOSE, 3 = ISSUE_REF, 4 = COMMIT_REF,
-		5 = COMMENT_REF, 6 = PULL_REF, 7 = COMMENT_LABEL, 12 = START_TRACKING,
+		5 = COMMENT_REF, 6 = PULL_REF, 7 = COMMENT_LABEL, 8 = MILESTONE_CHANGE,
+		9 = ASSIGNEES_CHANGE, 10 = TITLE_CHANGE, 11 = DELETE_BRANCH, 12 = START_TRACKING,
 		13 = STOP_TRACKING, 14 = ADD_TIME_MANUAL, 16 = ADDED_DEADLINE, 17 = MODIFIED_DEADLINE,
 		18 = REMOVED_DEADLINE, 19 = ADD_DEPENDENCY, 20 = REMOVE_DEPENDENCY, 21 = CODE,
 		22 = REVIEW, 23 = ISSUE_LOCKED, 24 = ISSUE_UNLOCKED, 25 = TARGET_BRANCH_CHANGED,
@@ -184,10 +185,9 @@
 					{{if gt .OldMilestoneID 0}}{{if gt .MilestoneID 0}}{{ctx.Locale.Tr "repo.issues.change_milestone_at" (.OldMilestone.Name|Escape) (.Milestone.Name|Escape) $createdStr | Safe}}{{else}}{{ctx.Locale.Tr "repo.issues.remove_milestone_at" (.OldMilestone.Name|Escape) $createdStr | Safe}}{{end}}{{else if gt .MilestoneID 0}}{{ctx.Locale.Tr "repo.issues.add_milestone_at" (.Milestone.Name|Escape) $createdStr | Safe}}{{end}}
 				</span>
 			</div>
-		{{else if eq .Type 9}}
+		{{else if and (eq .Type 9) (gt .AssigneeID 0)}}
 			<div class="timeline-item event" id="{{.HashTag}}">
 				<span class="badge">{{svg "octicon-person"}}</span>
-				{{if gt .AssigneeID 0}}
 				{{if .RemovedAssignee}}
 					{{template "shared/user/avatarlink" dict "user" .Assignee}}
 					<span class="text grey muted-links">
@@ -209,7 +209,6 @@
 						{{end}}
 					</span>
 				{{end}}
-				{{end}}
 			</div>
 		{{else if eq .Type 10}}
 			<div class="timeline-item event" id="{{.HashTag}}">