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

tests/phpunit/tests/admin/includesPlugin.php

@@ -292,6 +292,13 @@ class Tests_Admin_includesPlugin extends WP_UnitTestCase {
 		$this->assertTrue( isset( $result[ $plugin[ 0 ] ] ) );
 	}
 
+	/**
+	 * @covers ::validate_active_plugins
+	 */
+	public function test_validate_active_plugins_empty() {
+		$this->assertEquals( array(), validate_active_plugins() );
+	}
+
 	/**
 	 * @covers ::is_uninstallable_plugin
 	 */