Symfony version(s) affected: 5.0.5

Description
When using typed properties in a class, the serializer correctly skips those that are not initialized yet. However, when serializing a collection of objects that are instances of the same class, the serializer's attribute caching kicks in, which may lead to "Typed property must not be accessed before initialization" errors, e.g. when property "a" of object 1 is initialized, but on object 2 it is not.

How to reproduce

class Test {
    public string $a;
    public string $b;
}

$test1 = new Test();
$test1->a = "a";
$test1->b = "b";

$test2 = new Test();
$test2->a = "a";

$objects = [$test1, $test2];

$serializer->serialize($objects, "json");

(where $serializer is the default serializer service that is configured by Symfony when enabling the serializer component)

Possible Solution
The "allowed" status of typed properties must not be cached and checked every time an object is serialized.