Fallback to the default localization resource for base classes with the L property

[hikalkan]

Base classes like ApplicationService has a L property. But it requires to set the LocalizationResource property by the derived class. Otherwise, it fails. We generally set it in the constructor of the derived class (example).
If the developer inherits from this class, no problem. But sometimes developers directly inherit from to ApplicationService and forgot to set the resource type. In such cases, we will try to fallback to the default resource (introduced with #3790 and documented here). If developer set it (with is already done in the startup template) everyting will work as expected.