If you want to prompt the user to provide some valid java package you'd specify PromptType.JAVA_PACKAGE. Unfortunately this prompt type considers the empty string as invalid package even though I'd argue that this is the default package.
If I ask the user for the package, scaffolded resources would get created to, I'd do:

prompt.promptCommon(target package: ", PromptType.JAVA_PACKAGE, project.getFacet(JavaSourceFacet.class).getBasePackageResource().toString())

If I have no valid packageResource (ex. forge would return an invalid base package) I'd want to suggest the user to put his sources into the default package (for a maven project: src/main/java). I'd then feed a "" as default to the prompt:

prompt.promptCommon(target package: ", PromptType.JAVA_PACKAGE, "")

Unfortunately the PromptType.JAVA_PACKAGE would not validate the empty string. It would tell me that this is not a valid package.