return null for default dataDir

src/apps.js

@@ -406,6 +406,8 @@ function getDuplicateErrorDetails(errorMessage, location, domainObject, portBind
 }
 
 function getDataDir(app, dataDir) {
+    assert(dataDir === null || typeof dataDir === 'string');
+
     return dataDir || path.join(paths.APPS_DATA_DIR, app.id, 'data');
 }