[BUGFIX beta] Prevent error in LOG_RESOLVER during unit testing.

When using `ember-qunit` to unit test a component, an error is thrown
since there is no `root` object (i.e. no application is booted).
Merge pull request #4950 from rjackson/allow-null-root-for-log-resolver

[BUGFIX beta] Prevent error in LOG_RESOLVER during unit testing.