Returns Optional's in DAO's/Repositories

[mzudse]

We could return a Optional instead of a object directly. Then we could remove the null checks and improve the code readability.

userEntity.isPresent() // userEntity.isEmpty()
vs
userEntity != null // userEntity == null

public UserEntity findByEmail(final String email) {
    return find("email", email).singleResult();
}

public Optional<UserEntity> findByEmail(final String email) {
    return Optional.ofNullable(find("email", email).singleResult());
}

The PanacheRepositoryBase already contains the method 'findByIdOptional' as an alternative of 'findById'

[astanik]

This is a great idea, if we only talk about the return types of repositories. This would only affect the null checks in the controller layer.

This discussion can be implemented with #90