allow app/templates to not exist when using pod structure

When using the pod file structure, you won't necessarily need `./app/templates`. If you don't have it, ember-cli fails to build your app. This PR makes that path optional.

Before this change, the new test fails with:

```
Building
Building.
Build failed.

ENOENT, no such file or directory 'app/templates/'

Error: ENOENT, no such file or directory 'app/templates/'
```

After this change, the test passes.

Does this deserve a changelog entry? If so, I'll add it.