LibJS: Make Array.prototype.at return undefined on empty slot

Author: https://github.com/davidot
Commit: https://github.com/SerenityOS/serenity/commit/2d87d5dab91
Pull-request: https://github.com/SerenityOS/serenity/pull/8025
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp

@@ -1486,7 +1486,7 @@ JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::at)
     }
     if (index.has_overflow() || index.value() >= length)
         return js_undefined();
-    return this_object->get(index.value());
+    return this_object->get(index.value()).value_or(js_undefined());
 }
 
 }