Ensure empty layers are written to the index file

[philwebb]

Raised on Gitter:

I noticed that from 2.3.0.M1 to 2.3.0.M4 the behavior has changed slightly. Now, a snapshot layer is not created if the project does not have snapshot dependencies, which is cool, but also complicates the Dockerfile that needs to be created as it needs to know whether the specific application has snapshot dependencies or not. I can no longer use a generic Dockerfile. I can workaround this problem via a Docker COPY trick, but I wondered if this is a “conscious” change.

[nickstolwijk]

Will this solve https://stackoverflow.com/questions/61413181/spring-boot-2-3-0-m4-layered-jar-is-missing-a-layer-docker-fails-on-missing-dir/61413361 ?

[wilkinsona]

Yes, it will.