Hide diff stats on empty PRs (#30629) (#30645)

Backport #30629 by @silverwind

When a PR is empty, e.g. has neither additions nor deletions, we don't
need to show this:

<img width="125" alt="Screenshot 2024-04-21 at 23 25 38"
src="https://github.com/go-gitea/gitea/assets/115237/0b987eb5-66f5-4b9b-b5aa-7e9e267e9b52">

Co-authored-by: silverwind <me@silverwind.io>

templates/repo/pulls/tab_menu.tmpl

@@ -15,12 +15,14 @@
 			{{ctx.Locale.Tr "repo.pulls.tab_files"}}
 			<span class="ui small label">{{if .NumFiles}}{{.NumFiles}}{{else}}-{{end}}</span>
 		</a>
+		{{if or .Diff.TotalAddition .Diff.TotalDeletion}}
 		<span class="item tw-ml-auto tw-pr-0 tw-font-bold tw-flex tw-items-center tw-gap-2">
 			<span><span class="text green">{{if .Diff.TotalAddition}}+{{.Diff.TotalAddition}}{{end}}</span> <span class="text red">{{if .Diff.TotalDeletion}}-{{.Diff.TotalDeletion}}{{end}}</span></span>
 			<span class="diff-stats-bar">
 				<div class="diff-stats-add-bar" style="width: {{Eval 100 "*" .Diff.TotalAddition "/" "(" .Diff.TotalAddition "+" .Diff.TotalDeletion "+" 0.0 ")"}}%"></div>
 			</span>
 		</span>
+		{{end}}
 	</div>
 	<div class="ui tabs divider"></div>
 </div>