packer: Build only adds post-processor artifact if not nil

packer/build.go

@@ -175,7 +175,9 @@ PostProcessorRunSeqLoop:
 				continue PostProcessorRunSeqLoop
 			}
 
-			artifacts = append(artifacts, artifact)
+			if artifact != nil {
+				artifacts = append(artifacts, artifact)
+			}
 		}
 	}
 

packer/post_processor_test.go

@@ -18,5 +18,5 @@ func (pp *TestPostProcessor) PostProcess(ui Ui, a Artifact) (Artifact, error) {
 	pp.ppCalled = true
 	pp.ppArtifact = a
 	pp.ppUi = ui
-	return nil, nil
+	return new(TestArtifact), nil
 }