[DOC] Include `Ember.isEmpty({})` example

Coming from rails, I expected `Ember.isEmpty({})` to return `true`.
Since this isn't the case, it might be a good idea to illustrate this in
the docs, since there is already a unit test covering this case.
Merge pull request #9709 from seanpdoyle/hash-is-empty

[DOC] Include `Ember.isEmpty({})` example