Skip empty page template meta

The check for `false !== $custom_page_template` is too strict as `get_post_meta` returns an empty string rather than `(bool) false`. This incorrectly caused the `$custom_page_template` if statement to return true which then caused a `locate_template( array( '' ) );` further down which would fail thus causing pages to fall back to using `single.php` instead of `page.php`.

This PR loosens the `$custom_page_template` check so that any falsey value will cause a fallback to `page.php`.

The actual code change is in c8f09ce in-case you want just that and not the associated whitespace cleanup in 6f908e1

Would be great to get this merged as it's affecting a current project.