Redundant post ID validation

[SergeyBiryukov]

We have this in some of the recently introduced functions:

$post = empty( $post_id ) ? get_post() : get_post( $post_id );

That kind of validation is redundant, it duplicates the check in get_post(). We can just always call get_post( $post_id ) instead.

[ryan]

[23772]

[ryan]

In 24249:

Remove redundant post ID validation.

Props SergeyBiryukov
fixes #24199