Fix possibility for "Undefined index: class" assigning .stripe-payment-form in hook_buildForm().

stripe.php

@@ -122,6 +122,9 @@ function stripe_civicrm_buildForm($formName, &$form) {
     if (!stristr($formName, '_Confirm') && !stristr($formName, '_ThankYou')) {
       // This is the 'Main', or first step of the form that collects CC data.
       if (!isset($form->_elementIndex['stripe_token'])) {
+        if (empty($form->_attributes['class'])) {
+          $form->_attributes['class'] = '';
+        }
         $form->_attributes['class'] .= " stripe-payment-form";
         $form->addElement('hidden', 'stripe_token', NULL, array('id' => 'stripe-token'));
         stripe_add_stripe_js($form);