fix public folder deleted breaking build

If the public folder is deleted, it breaks the build command.

Steps to reproduce in 0.0.39:

```sh
ember new public-deleted
cd public-deleted
rm -r public
ember build
```

Should throw an error like:
```
version: 0.0.39
Build failed.
ENOENT, no such file or directory 'public/'
Error: ENOENT, no such file or directory 'public/'
  at Object.fs.readdirSync (fs.js:654:18)
  at walkSync (/Users/jake/Work/ember-cli-cordova-auth/test-app/node_modules/ember-cli/node_modules/walk-sync/index.js:14:20)
  at /Users/jake/Work/ember-cli-cordova-auth/test-app/node_modules/ember-cli/node_modules/broccoli-merge-trees/index.js:26:26
  at tryCatch (/Users/jake/Work/ember-cli-cordova-auth/test-app/node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/-internal.js:163:16)
  at invokeCallback (/Users/jake/Work/ember-cli-cordova-auth/test-app/node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/-internal.js:172:17)
  at publish (/Users/jake/Work/ember-cli-cordova-auth/test-app/node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/-internal.js:150:13)
  at flush (/Users/jake/Work/ember-cli-cordova-auth/test-app/node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/asap.js:51:9)
  at process._tickCallback (node.js:415:13)
```