[BUGFIX beta] Fix bound helper argument types

Fix SimpleHandlebarsView.prototype.normalizedValue's handling of null
and undefined cases (#9280).

The issue was introduced by this commit (handlebars_bound_view.js):
https://github.com/emberjs/ember.js/compare/b9e6166...b87696b#diff-3
Merge pull request #9389 from brzpegasus/bound-helper-arg-type-fix

[BUGFIX beta] Fix bound helper argument types