Fixed min/max date validation in DatePicker
Fixed min/max date validation in DatePicker

The DatePicker ended up showing the min/max date validation error when
the the min or max date was null while the the other was defined. any
date is considered greater than null, so it threw an error. It will only
throw an error if both are defined and true-ish now.

Closes #293

mlaursen/react-md,mlaursen/react-md,mlaursen/react-md,lwhitlock/grow-tracker