[DUOS-2851][risk=no] Minor safety check for studies with no datasets

[rushtong]

Addresses

https://broadworkbench.atlassian.net/browse/DUOS-2851

Summary

Minor null check to capture studies with empty datasets

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[hams7504]

looks good 👍

[okotsopoulos]

If you'd like, here is another way to handle this (we like ListUtils.emptyIfNull in TDR for cases like this):

import org.apache.commons.collections4.ListUtils;

…

Optional<Dataset> dataset =
  ListUtils.emptyIfNull(existingStudy.getDatasets())
    .stream()
    .filter(d -> d.getDataSetId().equals(cg.getDatasetId()))
    .findFirst();

[rushtong]

Thank you, that does look cleaner!