Prevent Cannot read property '$promise' of undefined when null

[garrypas]

When I have properties that are that are null on my model everything work OK with Angular's ng-bind, but with bindonce I get an error like the following:

TypeError: Cannot read property '$promise' of undefined at r.controller.checkBindonce (bindonce.min.js:1) at bindonce.min.js:1 at m.$digest (angular.js:17855) at m.$apply (angular.js:18125) at l (angular.js:12233) at XMLHttpRequest.t.onload (angular.js:12387)

However with the code added in this PR that also checks for nulls this error no longer occurs and everything behaves as expected