[BUGFIX release] Don't assume that the router has a container.

As part of ember-flows-generator I `eval()` an entire second route file separate from the context of the currently running application. That router does not have an attached container object. As part of the 1.5.1 fixes we have one unprotected property access for `this.container.has()` which this PR fixes.

Tagged release so that it gets into that branch, but it doesn't need to trigger 1.5.2. Let me know if you'd prefer it retagged beta.