[cache] allow null value

[celestialaly]

Actual behavior

When we call the findOneBy method:

• if there is no result, we put in cache a null value
• we allow a null value in the saveToCache method
  • if the cache key exists, we throw a RuntimeException if the value is null (in the fetchFromCache method)

New behavior

Considering the fact that the cache key is specific enough (ex: oauth_mapado_rest_client__v1_customer_statuses?label=Etudiant&contract=%2Fv1%2Fcontracts%2F237&fields=id%2Clabel), I think we can allow null values in the cache.

[JulienRAVIA]

Thanks @celestialaly for your contribution 💪💪