Multisite: Require a site title when a site is created

The UI for adding a site says that title is required, however the actual code wasn't requiring it.

Props ideag.
Fixes #54012.


git-svn-id: https://develop.svn.wordpress.org/trunk@55894 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/network/site-new.php

@@ -83,6 +83,10 @@ if ( isset( $_REQUEST['action'] ) && 'add-site' === $_REQUEST['action'] ) {
 		}
 	}
 
+	if ( empty( $title ) ) {
+		wp_die( __( 'Missing site title.' ) );
+	}
+
 	if ( empty( $domain ) ) {
 		wp_die( __( 'Missing or invalid site address.' ) );
 	}