support for projects having no Configurations

[mathjeff]

If a project doesn't have any dependencies, it would seem that the project wouldn't need to pass any Configurations to the plugin, but actually if you do that you get a message saying

Document must have at least one relationship of type DOCUMENT_DESCRIBES

and also the resulting spdx file doesn't mention DOCUMENT_DESCRIBES in it

It would be nice to support projects having no Configurations, even if it means the plugin generates and adds an empty Configuration

[mathjeff]

in the meanwhile, a workaround seems to be to add an empty Configuration in the project and pass it to the plugin