Replace getPersistentEntity with getRequiredPersistentEntity

[roimenashe]

Replace getPersistentEntity with getRequiredPersistentEntity in places where potentially NullPointerException can be thrown.

[roimenashe]

@Aloren
I kept the getPersistentEntity in places where there are null handling (MappingAerospikeReadConverter convertCustomType method line 154)
And different “getPersistentEntity” method that serves as a getter (BaseAerospikePersistenceEntityIndexCreator onApplicationEvent method lines 55 and 63).

Every getPersistentEntity that could result in NPE (even in tests) as the one you mentioned was replaced with getRequiredPersistentEntity which throws the relevant exception.