Avoid null artifacts when generating module IDs

[efegokdemir]

Summary

• Avoid dereferencing an unresolved module artifact while generating deployment descriptor IDs.
• Leave the module element without a generated ID until its artifact is available.
• Add regression coverage for unresolved modules.

Testing

• mvn -Dspotless.check.skip=true -Dspotless.apply.skip=true test — passed (49 tests)
• git diff --check — passed

Fixes #512