[MJLINK-9] force NPE on empty sources.

[bmarwell]

If a separate dist project with packaging=jlink is being used, AND no sources other than a module-info.java are present, this will cause a NPE as indicated in the JIRA issue.

It is intended that his PR will result in a test failure unless the underlying issue is being fixed.

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[sormuras]

Thanks for the PR, Ben. I converted it to a draft -- needs a fix for the NPE to be merged. 🤡

[sormuras]

Superseded by a330205