RULE: null concepts in datapackage

[angiehjort]

Should trigger error
(example here)

[jheeffer]

For entities value: null can be valid. Though for datapoints, like here, indeed invalid.

https://docs.google.com/document/d/10yzpTirAe-sAeMoCRzmMiCde1fEAtzMR6zfRY5E-fQs/edit#heading=h.dgrzyx4oa909