convert null to an empty map/set
convert null to an empty map/set

We already do this for lists, it makes more things work without
downside that I can think of.

On the upside mongoDB can intentionally store nothing at all for the field
when then Set/Map is empty, saving disk space.

gbwey/persistent,jcristovao/persistent,jasonzoladz/persistent,jasonzoladz/persistent,plow-technologies/persistent,erikd/persistent,gbwey/persistent,gbwey/persistent,andrewthad/persistent,jcristovao/persistent,pseudonom/persistent,mitchellwrosen/persistent,rdnetto/persistent,nakaji-dayo/persistent,andrewthad/persistent,bitemyapp/persistent,meteficha/persistent,rdnetto/persistent,superduper/persistent,greydot/persistent,greydot/persistent,psibi/persistent,meteficha/persistent,paul-rouse/persistent,mitchellwrosen/persistent,pseudonom/persistent,junjihashimoto/persistent,naushadh/persistent,superduper/persistent,greydot/persistent,mkscrg/persistent,superduper/persistent,yesodweb/persistent,nakaji-dayo/persistent,creichert/persistent,mitchellwrosen/persistent,nakaji-dayo/persistent,mkscrg/persistent,meteficha/persistent,andrewthad/persistent,jasonzoladz/persistent,junjihashimoto/persistent,mkscrg/persistent,jcristovao/persistent