Infinite Scroll: Settings label reads "Shows 0 posts on each load"

Under /wp-admin/options-reading.php, the label for Infinite Scroll now reads "Shows 0 posts on each load" when you're using a theme that does not explicitly set posts-per-page in the infinite scroll settings. It used to say the default, "Shows 7 posts on each load".

#### Steps to reproduce the issue

1. Start with a theme that does not set posts-per-page in the Infinite Scroll settings (like Apostrophe 2)
2. Navigate to Dashboard > Settings > Reading
3. Look at the infinite scroll settings.

#### What I expected
To see "Shows 7 posts on each load"

#### What happened instead
It read "Shows 0 posts on each load". Infinite scroll does work as expected though!

If you use a theme that does set posts-per-page in the infinite scroll settings (like Sketch or Dyad), the correct number is displayed.

I can recreate this on an AT site and a self-hosted site. On WordPress.com, this setting is replaced with a link to the Calypso interface.

This was originally reported as a theme issue here: https://github.com/Automattic/themes/issues/260 (hattip @druesome).

* **User report:** https://en.forums.wordpress.com/topic/my-home-page-wont-load-everything/
* **User's site:** https://travelswiththecrew.com/

Start at the WordPress Settings > Reading page at /wp-admin/options-reading.php and trace how Infinite Scroll determines the posts-per-page label when a theme provides no value. Reproduce the issue with a theme such as Apostrophe 2, then verify that the default label shows 7 posts while theme-provided values still display correctly.

Written by the indexing model from the issue text.