Add `__return_null()` to complement the existing true, false, zero, and empty...

Add `__return_null()` to complement the existing true, false, zero, and empty array methods. props johnbillion, fixes #18845.



git-svn-id: https://core.svn.wordpress.org/trunk@19607 1a063a9b-81f0-0310-95a4-ce76da25c4cd