fixed issues with empty Discrete1DKDE

we were getting some issues when the Discrete1DKDE had no samples in it (nans from dividing by len(self._obs) and invalid beta-distribution parameters). This should be a corner case that is not encountered often, probably only during initial start-up of streaming processes.

relatively low priority since our production runs are using FixedBandwidth1DKDE, but would be good to get in at some point. Probably doesn't warrant redeploying the production runs when this goes in; we can just let it tag along whenever we have something else we'd like to push.