`Ember.TargetActionSupport`'s `sendAction` should support `null` as context

Doing:

```javascript
this.sendAction('someAction', null);
```

in e.g. components do not have the expected behavior = that `null` is received as the context in the other end. Instead, since `opts['actionContext']` is not truthy, the `actionContextObject` property or `this` would be used as a default.

I also shaved off a few bytes by using `.property` instead of `['property']`.