Check for existence of pagenow global

[KalenJohnson]

There are some plugins that call home_url() or get_home_url() without being in any type of hook, so it's happening right when the plugin get's loaded. It doesn't seem like this is always an issue, but I am seeing many warnings of an undefined index.

This patch I don't believe will break anything, since if things are being loaded before the $pagenow global exists, this check won't work anyways, so checking for it and removing a PHP warning seems like a win.

[SergeyBiryukov]

Introduced in [24844].

[wonderboymusic]

In 33736:

In get_home_url(), import the $pagenow global to avoid having to check if it exists before comparing against it.

Props KalenJohnson.
See #33545.

[KalenJohnson]

<3