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

src/wp-admin/includes/plugin.php

@@ -884,7 +884,7 @@ function validate_active_plugins() {
 	}
 
 	if ( empty( $plugins ) )
-		return;
+		return array();
 
 	$invalid = array();