Fix template type error on SSO clients page

{{if not .Clients}} fails when .Clients is a non-nil empty slice
because Go templates expect a bool operand for 'not'. Use
{{if eq (len .Clients) 0}} instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/sso_clients.html

@@ -49,6 +49,6 @@
       {{end}}
     </tbody>
   </table>
-  {{if not .Clients}}<p class="text-muted" style="text-align:center;padding:2rem">No SSO clients registered.</p>{{end}}
+  {{if eq (len .Clients) 0}}<p class="text-muted" style="text-align:center;padding:2rem">No SSO clients registered.</p>{{end}}
 </div>
 {{end}}