Dates returning as 'null' in JSON

Some Crossref DOI records have their issued date-parts set to null, such as 10.22541/au.149693987.70506124. 

This is causing failures of clients that expect there to be either an empty array or valid date components.

Other example data: "issued": { "date-parts": [[null]] },

Example DOIs with this issue: 
10.1371/journal.pone.0162471.g003 
10.7554/elife.22707.006