get_queried_object_id() method in pre_get_posts

[wpsmith]

When using $wp_query->get_queried_object_id() within the pre_get_posts hook results in an error:
Notice: Undefined property: WP_Query::$post in C:\xampp\htdocs\wp\wp-includes\query.php on line 3700

Here is a simple test:
​https://gist.github.com/wpsmith/0c5a1d497eb5738309f6

[wpsmith]

query.php patch

[kovshenin]

An alternative approach in 28412.diff​: since we're checking for null with is_null(), let's actually set it to null during init.

[SergeyBiryukov]

In 28667:

Avoid a PHP notice when using WP_Query::get_queried_object() in pre_get_posts action.

props wpsmith.
fixes #28412.