Empty developer.wordpress.org post_type[] results should return a 404 status

[jonoaldersonwp]

E.g., ​https://developer.wordpress.org/?post_type[]=cats

[coffee2code]

In 9316:

Developer: 404 any attempts to filter by invalid post type.

Props jonoaldersonwp, coffee2code.
Fixes #4647.

[dd32]

FWIW This should probably be fixed in core too, since it looks like redirect_canonical handles regular urls such as ​https://developer.wordpress.org/?post_type=cats

[dd32]

In 9369:

Developer: Avoid a PHP Warning on ?post_type= requests.

See #4647.