ContentPackage2FeatureModelConverter.getAclManager should never return null

ContentPackage2FeatureModelConverter.getAclManager is used across the converter code base and always expected to be not-null. so, instead of defining it as 'nullable' we should find ways to make sure it is always present on the converter. this would allow to get rid of Objects.requireNotNull and the warnings about potential NPE where the check is missing.