bug symfony#27848 [Workflow] Fixed BC break (lyrixx)

This PR was merged into the 4.1 branch.

Discussion
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[Workflow] Fixed BC break

| Q             | A
| ------------- | ---
| Branch?       | 4.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#27759
| License       | MIT
| Doc PR        |

Commits
-------

b24acb0 [Workflow] Fixed BC break

Author: nicolas-grekas

src/Symfony/Component/Workflow/Event/Event.php

@@ -40,7 +40,10 @@ public function __construct($subject, Marking $marking, Transition $transition,
         $this->subject = $subject;
         $this->marking = $marking;
         $this->transition = $transition;
-        if (is_string($workflow)) {
+        if (null === $workflow) {
+            @trigger_error(sprintf('Passing only three parameters to "%s" is deprecated since Symfony 4.1. Pass a %s instance as fourth parameter instead.', __METHOD__, WorkflowInterface::class), E_USER_DEPRECATED);
+            $this->workflowName = 'unnamed';
+        } elseif (is_string($workflow)) {
             @trigger_error(sprintf('Passing a string as 4th parameter of "%s" is deprecated since Symfony 4.1. Pass a %s instance instead.', __METHOD__, WorkflowInterface::class), E_USER_DEPRECATED);
             $this->workflowName = $workflow;
         } elseif ($workflow instanceof WorkflowInterface) {