Add some deprecations about invalid values

[franmomu]

Subject

Add some deprecations at ModelManager in order to be prepared for the next major.

It's based on sonata-project/SonataDoctrineORMAdminBundle#1049

This would make PHPStan check pass in #450

I am targeting this branch, because these changes are BC.

Changelog

### Deprecated
- Deprecated passing `null` as argument 2 for `ModelManager::find()`;
- Deprecated passing `null` as argument 1 for `ModelManager::getNormalizedIdentifier()`;
- Deprecated passing other type than `object` as argument 1 for `ModelManager::getUrlSafeIdentifier()`;

[phansys]

Thank you @franmomu!