fix ember init command in empty directory

Fixes #1759

As mentioned in the issue:

There is a test for `ember init` in an empty directory, but it is being contaminated by ember-cli itself. The temp directory created by the test is a descendant of the ember-cli directory, so when `Project.closest` is called it walks through the temp directory's ancestors until it finds `package.json` in the ember-cli root. Consequently `NULL_PROJECT` is not used by the test.