Revert "null builder: say that we are returnin the ID value"

This reverts commit 8e923977f0.

builder/null/builder.go

@@ -25,7 +25,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 		return nil, warnings, errs
 	}
 
-	return []string{"ID"}, warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {