Fix notice for function wporg_is_handbook on single post pages

[keesiemeijer]

The function wporg_is_handbook() displays this notice on single post pages when used before the queried object is set (as in the pre_get_posts action).

Notice: Trying to get property of non-object in /srv/www/wp-reference/public/wp-includes/query.php on line 4654

This notice originates from WP_Query::is_singular() where get_queried_object() is used.

To fix this warning there needs to be a check if the queried object is set. If it's not set, use the post type query var from the current query to determine if it's a handbook page.

[keesiemeijer]

Fix notice

[coffee2code]

In 3262:

Handbook plugin: Permit use of wporg_is_handbook() without warnings before main query runs.

Props keesiemeijer.
Fixes #1731.