allow execution if no ignoredModules or legactyFilesToAppend are specified

If I do not specify legacyFilesToAppend or ignoredModules then I will get unhelpful exceptions:
```
  var componentJs = compileES6(componentAndVendorTree, {
    loaderFile: 'loader.js',
    inputFiles: [
      'dist/**/*.js'
    ],
    wrapInEval: true,
    outputFile: '/dist/ember-metions.js'
  });

```

I need to change the above to this:

```
  var componentJs = compileES6(componentAndVendorTree, {
    loaderFile: 'loader.js',
    legacyFilesToAppend: [],
    ignoredModules: [],
    inputFiles: [
      'dist/**/*.js'
    ],
    wrapInEval: true,
    outputFile: '/dist/ember-metions.js'
  });

```
The PR makes this unnecessary.  It is either this or it changes to a more helpful error message?