Upgrade/Install: Cast get_option( 'active_plugins' ) to array in upgrade_690().

This resolves an error on Multisite networks where an individual site doesn't have plugins active:
{{{
array_search(): Argument #2 ($haystack) must be of type array, string given
}}}

Follow-up to [60666], [60721].

Props dd32.
See #53323.

git-svn-id: https://develop.svn.wordpress.org/trunk@60725 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/upgrade.php

@@ -2498,7 +2498,7 @@ function upgrade_690() {
 
 	if ( $wp_current_db_version < 60717 ) {
 		// Switch Hello Dolly from file to directory format. See #53323
-		$active_plugins = get_option( 'active_plugins' );
+		$active_plugins = (array) get_option( 'active_plugins', array() );
 		$old_plugin     = 'hello.php';
 		$new_plugin     = 'hello-dolly/hello.php';
 		$key            = array_search( $old_plugin, $active_plugins, true );