fix(tooltip): null scope check in isOpen watch

[RobJacobs]

The watch for the isOpen property from the model was not
checking to see if the internal tooltip scope was still
valid (null or undefined) before comparing the value to
the tooltip scope isOpen property.

Closes #4697
Fixes #4683

[RobJacobs]

I'm not sure how to add a test for this. The scenario is the element with the tooltip attached has it's scope destroyed and falls into the scope.$on('$destroy' function where ttScope is set to null. The person reporting the problem (with no plunk) claims an error is being thrown (Cannot set property 'isOpen' of undefined) in the watch on the element scope isOpen here. How would I set the element scope isOpen property to trigger the call to the watch function after the element scope is destroyed?