[BUGFIX beta] Enable @ember/object#get to be called with an empty string

[nlfurniss]

In plain JS, this works just fine

let myObj = {
  '': 'true'
};

myObj['']; // true

However Ember throws an error if you do: get(myObj, ''). Spoke with @chancancode and he told me that previously there was a bug where calling get with an empty string would return myObj. However, this has been fixed at some point and it seems like this should work in Ember.

• Remove keyName !== '' assertion
• Add test to ensure calling get with empty string returns a value if the key '' exists
• Add test to ensure calling get with empty string returns undefined if the key '' doesn't exist
• Add tests to ensure {{each-in}} renders items when the hash has a key of empty string

[chancancode]

r? @rwjblue