compare: NPE when buildinfo length/sha512 entry is missing

In `CompareMojo.checkArtifact()` (line 291):

```java
} else if (!actualLength.equals(referenceLength)) {
```

`actualLength` comes from `actual.remove(prefix + ".length")` and can be `null` when the freshly generated buildinfo is missing a `length` entry (for example after printer issues). In that case `actualLength.equals(...)` throws a `NullPointerException` instead of reporting the artifact as mismatched/missing.

Same applies to `actualSha512` (line 293).

Suggested fix: use `Objects.equals(actualLength, referenceLength)` / `Objects.equals(actualSha512, referenceSha512)`.

Start in CompareMojo.checkArtifact() around lines 291-293 and trace how actualLength and actualSha512 are removed from the generated buildinfo data. Exercise cases where either entry is missing, then verify the comparison reports the artifact as mismatched or missing without throwing a NullPointerException.

Written by the indexing model from the issue text.