fix(nav): guard postNav against nil main menu

[jjsnack]

Summary

• `postNav.html` computed `len($menus)` directly, which panics when `Site.Menus.main` is nil (e.g. `404.html`, or any content-less/no-menu site).
• `nav.html` was already patched for this in `fix/nil-menu-guard` ("Guard nav against nil menu on content-less sites"), but `postNav.html` was added afterward without the same guard.
• Mirrors the existing `with`-guarded pattern from `nav.html`.

Test plan

• `hugo --gc --minify -D` in a site with no `main` menu configured now builds clean (was previously failing with error calling len: reflect: call of reflect.Value.Type on zero Value in postNav.html)