productionTime getter throws for projects not in production

[robireton]

REDCap returns production_time: "" for development-mode projects, and REDCapDatetime.parse throws a TypeError on anything that isn't exactly 19 characters (lib/datetime.js:24). So reading project.info.productionTime (lib/info.js:72-74) crashes instead of returning null like the other empty-string getters in this class. creationTime has the same exposure if the field is ever empty.

Fix: return null when the underlying value is an empty string.