In `multi_resize()` image editor methods, assert that `null` can only be passed for one of the arguments, not both. Add a lot more unit test assertions to ensure this.

Props pbearne, DH-Shredder.
Fixes #26823.



git-svn-id: https://develop.svn.wordpress.org/trunk@27794 602fd350-edb4-49c9-b593-d223f7449a82