Allow/fix review (approve/reject) of empty PRs (#25690)

gitea allows to create empty PRs.

Currently when you need approvals for a merge, you have to manually add
/files to the url to get to the files tab to approve / reject the PR.

This PR allows to open the files tab via the normal tab / link and then
fixes the layout of the files tab.

**Screenshots:**

Before:

![image](b5082e5e-8c32-4412-993e-b854905e96d3)

After:

![image](1f5e056e-396f-4dfb-8d14-e17a2f6495d9)

---------

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

templates/repo/pulls/tab_menu.tmpl

@@ -9,7 +9,7 @@
 		{{$.locale.Tr "repo.pulls.tab_commits"}}
 		<span class="ui small label">{{if .NumCommits}}{{.NumCommits}}{{else}}-{{end}}</span>
 	</a>
-	<a class="item {{if .PageIsPullFiles}}active{{end}}" {{if .NumFiles}}href="{{.Issue.Link}}/files"{{end}}>
+	<a class="item {{if .PageIsPullFiles}}active{{end}}" href="{{.Issue.Link}}/files">
 		{{svg "octicon-diff"}}
 		{{$.locale.Tr "repo.pulls.tab_files"}}
 		<span class="ui small label">{{if .NumFiles}}{{.NumFiles}}{{else}}-{{end}}</span>