When no plugins are active, return an empty array from `validate_active_plugins()`.

This creates parity with the behavior of the function when plugins *are* active,
but none are invalid. It also makes it possible to write unit tests for the
function.

Props sgrant.
Fixes #30860.

git-svn-id: http://develop.svn.wordpress.org/trunk@31003 602fd350-edb4-49c9-b593-d223f7449a82