DefaultArtifact.equals throws NullPointerException if o.version is null

DefaultArtifact.validateIdentity() only throws if BOTH version and versionRange are null, so it's possible to create an instance of DefaultArtifact where version == null. If such an instance ever ends up in DefaultArtifact.equals(o), the check a.getVersion().equals(version) will throw a NullPointerException.

(This was observed while debugging an issue in versions-maven-plugin)