Merge pull request #11329 from atom/tj-fix-nullref-callback

Fix potential null reference callback invokation in script/clean

script/utils/child-process-wrapper.js

@@ -17,7 +17,7 @@ exports.safeExec = function(command, options, callback) {
   var child = childProcess.exec(command, options, function(error, stdout, stderr) {
     if (error)
       process.exit(error.code || 1);
-    else
+    else if (callback)
       callback(null);
   });
   child.stderr.pipe(process.stderr);