The bug itself is explained in the following (simplified) example:

class Dummy
{
    public array $requiredData; // supposed to be set for any object
    public array $optionalData; // can be not initialized (and if so - ignored by serializer)
}

$object1 = new Dummy();
$object1->requiredData = ['username' => 'foo'];
$json1 = $serializer->serialize($object1, 'json'); // {"requiredData": {"username": "foo"}}
// at this point object normalizer has already cached attributes for Dummy::class and context,
// now it contains array ['requiredData'] - optionalData has been ignored as it's unitialized

// then, while the script is still running, we have another object of the same class with optionalData set
$object2 = new Dummy();
$object2->requiredData = ['username' => 'bar'];
$object2->optionalData = ['email' => 'bar@test.com'];
$json2 = $serializer->serialize($object2, 'json');
// expected: {"requiredData": {"username": "bar"}, "optionalData": {"email": "bar@test.com"}}
// actual: {"requiredData": {"username": "bar"}}
// here normalizer has no clue about optionalData attribute since it reuses attributes cached
// in \Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer::$attributesCache
// while normalizing the first object

Though this PR created for 5.4 branch, it actually fixes a bug reproducible in 5.3. The reason why I use 5.4 for this fix is that 5.4 introduces a new feature related to the same problem. If this PR gets approved and merged, I can potentially implement the same fix for 5.3, but SKIP_UNINITIALIZED_VALUES will cause some merge conflicts in the future..

As of v 5.3 symfony ignores uninitialized properties by default in ObjectNormalizer::extractAttributes and PropertyNormalizer::extractAttributes (implemented in #38900 and #34791) but this approach is wrong - extractAttributes method MUST return the same attributes list for any instance of the same class, otherwise cached attributes do not match actual attributes list for different instances having different set of initialized/uninitialized properties

So, this PR does a few things:

1. removes ignoring attributes from all built-in implementations of \Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer::extractAttributes so that even uninitialized attributes will be cached by normalizer. Instead, we ignore uninitialized attributes when trying to access them while calling \Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer::getAttributeValue
2. sets true as the default value for SKIP_UNINITIALIZED_VALUES context setting as I believe this is the expected default behavior for any built-in object normalizer
3. makes SKIP_UNINITIALIZED_VALUES compatible with not just ObjectNormalizer, but with two other built-in normalizers PropertyNormalizer and GetSetMethodNormalizer as well