Do not render empty comments (#29039)

Follow #28654

The `comments` might be empty, so the templates shouldn't (and couldn't)
use it to render. When there is no comment, the UI should also be
updated to empty, so returning an empty body is good enough.

routers/web/repo/pull_review.go

@@ -153,12 +153,19 @@ func UpdateResolveConversation(ctx *context.Context) {
 }
 
 func renderConversation(ctx *context.Context, comment *issues_model.Comment, origin string) {
+	ctx.Data["PageIsPullFiles"] = origin == "diff"
+
 	comments, err := issues_model.FetchCodeCommentsByLine(ctx, comment.Issue, ctx.Doer, comment.TreePath, comment.Line, ctx.Data["ShowOutdatedComments"].(bool))
 	if err != nil {
 		ctx.ServerError("FetchCodeCommentsByLine", err)
 		return
 	}
-	ctx.Data["PageIsPullFiles"] = (origin == "diff")
+	if len(comments) == 0 {
+		// if the comments are empty (deleted, outdated, etc), it doesn't need to render anything, just return an empty body to replace "conversation-holder" on the page
+		ctx.Resp.WriteHeader(http.StatusOK)
+		return
+	}
+
 	ctx.Data["comments"] = comments
 	if ctx.Data["CanMarkConversation"], err = issues_model.CanMarkConversation(ctx, comment.Issue, ctx.Doer); err != nil {
 		ctx.ServerError("CanMarkConversation", err)
@@ -179,6 +186,8 @@ func renderConversation(ctx *context.Context, comment *issues_model.Comment, ori
 		ctx.HTML(http.StatusOK, tplDiffConversation)
 	} else if origin == "timeline" {
 		ctx.HTML(http.StatusOK, tplTimelineConversation)
+	} else {
+		ctx.Error(http.StatusBadRequest, "Unknown origin: "+origin)
 	}
 }