FIx null return (#1010)

`Request::header` could possibly return null. Fix by adding default empty string return.

Author: phknot

src/Resolvers/UserAgentResolver.php

@@ -10,6 +10,6 @@ class UserAgentResolver implements Resolver
 {
     public static function resolve(Auditable $auditable): string
     {
-        return $auditable->preloadedResolverData['user_agent'] ?? Request::header('User-Agent');
+        return $auditable->preloadedResolverData['user_agent'] ?? Request::header('User-Agent', '');
     }
 }