Relax Blob.getInstance() to allow null objectName
Relax Blob.getInstance() to allow null objectName

Glacier support uses this method to represent vaults, which
have a size but not object name. Currently @Nonnull only results
in IDE warnings but eventually we want violations to fail builds. 
vladmunthiu/dasein-cloud-core,unwin/dasein-cloud-core,dasein-cloud/dasein-cloud-core,daniellemayne/dasein-cloud-core,jeffrey-yan/dasein-cloud-core,maksimov/dasein-cloud-core,mariapavlova/dasein-cloud-core,OSS-TheWeatherCompany/dasein-cloud-core,JanewzhWang/dasein-cloud-core,daniellemayne/dasein-cloud-core_old,drewlyall/dasein-cloud-core,TheWeatherCompany/dasein-cloud-core