Previously, when getting posts from the API with `sticky=true`, if there were no sticky posts set, the query would return all posts as if the `sticky` argument was not set. In this situation, the query should return an empty array instead.
A `sticky=true` query that should return an empty array (in the previous situation, or with `include` and no intersecting post IDs) was also broken in that it would query the post with ID 1.
Finally, this commit significantly improves test coverage for the `sticky` filter argument, including direct testing of the `WHERE` clauses generated by `WP_Query`.
Props ryelle.
Fixes#39947.
git-svn-id: https://develop.svn.wordpress.org/trunk@40122 602fd350-edb4-49c9-b593-d223f7449a82
e5d5f1058e