Revert "PHPStan, getName() never returns null so it can be removed from the return type."

This reverts commit 283f575.

Author: heinrichschiller

src/PhpSessionHandler.php

@@ -26,14 +26,9 @@ public function start(): void
     /**
      * {@inheritDoc}
      */
-    public function getName(): string|null
+    public function getName(): ?string
     {
-        $sessionName = session_name();
-        if ($sessionName === false) {
-            $sessionName = null;
-        }
-
-        return $sessionName;
+        return (string)session_name();
     }
 
     /**

src/SessionHandlerInterface.php

@@ -26,7 +26,7 @@ public function start(): void;
      *
      * @return string|null The name
      */
-    public function getName(): string|null;
+    public function getName(): ?string;
 
     /**
      * Get cookie params.