Get path set path falsy fixes

These are fixes for ENYO-3662, ENYO-3663, and another issue I saw while testing these where enyo.getData("<path-with-computed") would fail if a computed property had no dependencies.

I think these are all pretty insidious bugs and would like to get this into 2.3.0.  The code review needs special scrutiny... I've run unit tests and gone though by hand, but I want to make doubly sure the logic matches the original, minus bugs.

I didn't update CHANGELOG, as there are no API changes here, just more correct handling of the falsy cases, but I could be convinced otherwise that this should be noted.