allow undefined app.import environments

You can pass an object of environment keys to dependency values to `app.import` currently. If you set the value of an environment to null, the import for that depdendency is ignored.

I would like to be able to leave environments completely unspecified.

```javascript
app.import({development: 'vendor/pretender/pretender.js'});
```

But when you run `ember build --environment=production`, that currently throws:

```
You must pass a file to `app.import`.
For directories specify them to the constructor under the `trees` option.
```

I want to avoid specifying each environment I don't care about on each import statement. I would also not enjoy adding a new environment `, test: null` (or whatever) to all import statements.

This PR addresses the issue by changing the comparison from `assetPath === null` to `assetPath == null`. This allows `undefined` to match `null` for this check, allowing environments to go completely unspecified in import statements.

Does this sound acceptable?

---

*https://github.com/stefanpenner/ember-cli/issues/662 seems related.*

/cc @trek