DD-1486: NullPointerException for custom dataset terms

[jo-pol]

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[janvanmansum]

Closing this PR. The problem was present in Dataverse v5.14, but was already fixed in v6.0.

That said, the chained method call dsv.getDataset().getGlobalId().asString() is indeed still risky. It assumes that dsv, getDataset() and getGlobalId() can never return null. If this assumption is not warranted it would be a good idea have a separate method getPid. However, it should then not return no-doi-set but instead throw a more descriptive exception.