[BUGFIX beta] Prevent assertion when using view instead of template.

For reference see
https://github.com/emberjs/ember.js/commit/e4f0773c378239245583aa5062d509537e96e4a3.
Merge pull request #4017 from rjackson/relax_missing_template_assertion

[BUGFIX beta] Prevent assertion when using view instead of template.