[Workflow] Fixed initial places when no places are configured

I introduced a BC break in #30468 and this PR fix it.
With the full stack framework, when one does not configure the
initial_place(s) the DIC set `[]` for the initial values.
So it removes the initials values guessed in `Definition::addPlace()`

src/Symfony/Component/Workflow/Definition.php

@@ -94,7 +94,7 @@ final class Definition
 
     private function setInitialPlaces($places = null)
     {
-        if (null === $places) {
+        if (!$places) {
             return;
         }