Custom date field throwing DB constraint error "cannot be null"
I guess that shouldn't be a problem in set the column to null in the DB because those are custom field 

I did manage to do the same testing against lower version in 4.6.15 and mysql version 5.6.25 - Which works as excepted 

I guess this may be a issue with mysql version or in the code so just check with the lower version or wait till the next release and if you have the same issue then please raise an issue 

I guess this helps !!