[ament_copyright] Add missing license information to empty "copyrightable" files

I think `ament_copyright --add-missing` command should add license information to empty "copyrightable" files. This would allow writing snippets to create template files for example.

I noticed this case is particularly handled for source file types in this [section](https://github.com/ament/ament_lint/blob/master/ament_copyright/ament_copyright/main.py#L235-L266). What is the reason for this special treatment?