Resolve #3185869 "Trying to access array offset on value of type null"

Closes #3185869

workflow.module

@@ -269,7 +269,7 @@ function workflow_execute_transition(WorkflowTransitionInterface $transition, $f
  */
 function workflow_get_user_role_names($permission) {
   static $roles = NULL;
-  if ($roles[$permission]) {
+  if (isset($roles[$permission])) {
     return $roles[$permission];
   }