Avoid NPE in compare when the actual buildinfo has no length or sha512 entry

[SAY-5]

Closes #244

checkArtifact compared with actualLength.equals(referenceLength) and actualSha512.equals(referenceSha512), so a freshly generated buildinfo without a .length or .checksums.sha512 entry failed with a NullPointerException instead of reporting the artifact as mismatched. Move the three checks into a small helper that uses Objects.equals and add a unit test for the missing length case.

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