DatasetService::getLastUploadForDataset throws NPE if there were no uploads yet

[leostep]

As said in the name, DatasetService::getLastUploadForDataset throws NullPointerException in case no loads were performed yet.
The code tries to check if lastUploadUri is not empty

isEmpty(dataSet.getLastUploadUri())

but getLastUploadUri method actually returns lastUpload.uri of Dataset class. So when there were no uploads at all, lastUpload field is null and lastUpload.uri throws NPE