[BUGFIX] Do not assume null Ember.get targets are all globals

* Changes globals to only mean properties starting with $ or a
  capital letter.
* Changes normalizeTuple(obj, 'Foo') to resolve Foo as a global.
* Changes normalizeTuple(null, 'Foo') to resolve Foo as a global.
* Specs normalizeTuple('null', 'foo') to resolve to [undefined, ''].
  This is unfortunate but needed to keep getPath happy.

This code is old, and tricky to modify due to perf optimizations and
edge case behavior. It would be great to refactor more heavily at some
other date.

Fixes #3760