3.7 regression from [25119]: Have in_category() return false when the first argument is empty.

Adds unit tests.

props ericlewis.
fixes #25706 for trunk.

Built from https://develop.svn.wordpress.org/trunk@25923


git-svn-id: http://core.svn.wordpress.org/trunk@25882 1a063a9b-81f0-0310-95a4-ce76da25c4cd