fix null artifact

Author: mcanoy

src/main/java/com/redhat/labs/lodestar/service/MigrationService.java

@@ -120,7 +120,7 @@ private void migrateArtifacts() {
 
     private void migrateArtifactsToGitlab(Engagement engagement) {
         List<Artifact> artifacts = engagement.getArtifacts() == null ? Collections.emptyList() : engagement.getArtifacts();
-        String content = json.toJson(engagement.getArtifacts());
+        String content = json.toJson(artifacts);
         migrateToGitlab(engagement, content, ARTIFACT_JSON, artifacts.size());
 
     }