Check if the resource bundle inheritance works correctly in case a template set does not use its own bundle

If the template set does not use its on resource bundle, the getResourceBundle() is returning null right now and the handling can be wrong (NPE ?).
Check following use cases:

1) A template set has no parent template sets and does not provide any resource bundles.
In this case the jahiatemplates/common.properties should be the fallback bundle

2) A template set extends another template set and does not provide any resource bundles.
Resource bundle of the parent template set should be used as a fallback