Fix MU GlimmerResolver.normalize when fullName is not present

[ppcano]

This error is triggered when injecting an MU addon service like:

export default class ApplicationController extends Controller {
  @service('my-addon::hola') hola
}

The change checks that fullName is not empty to avoid triggering the error. The error is triggered when the registry calls normalize at https://github.com/emberjs/ember.js/blob/master/packages/%40ember/-internals/container/lib/registry.ts#L690:

In this case, options.source will be null, but options.namespace is my-addon.

      let normalizedFullName = this.normalize(fullName);
      let normalizedSource = this.normalize(options.source!);

This fix makes that the service can be resolved correctly.

I also wonder if the current implementation of the MU Glimmer.resolve may not cover all the possible cases, because the GlobalsResolver.resolve and ClassicResolver.resolve look different. (Note: I am not familiar with this code).

cc @rwjblue