Fix #6949: Stepper: onChangeStep function not initially set (#6982)

* Fix #6949: Stepper: onChangeStep function not initially set

* Update components/lib/stepper/StepperBase.js

---------

Co-authored-by: Melloware <[email protected]>

Author: meddvedev

components/lib/stepper/StepperBase.js

@@ -107,7 +107,8 @@ export const StepperBase = ComponentBase.extend({
         __TYPE: 'Stepper',
         activeStep: 0,
         orientation: 'horizontal',
-        linear: false
+        linear: false,
+        onChangeStep: null
     },
     css: {
         classes,