[12.x] Improved manager typehints (#57024)

Co-authored-by: Sergey Danilchenko <[email protected]>

Author: daniser

src/Illuminate/Session/SessionManager.php

@@ -269,7 +269,7 @@ public function getSessionConfig()
     /**
      * Get the default session driver name.
      *
-     * @return string
+     * @return string|null
      */
     public function getDefaultDriver()
     {

src/Illuminate/Support/Manager.php

@@ -50,7 +50,7 @@ public function __construct(Container $container)
     /**
      * Get the default driver name.
      *
-     * @return string
+     * @return string|null
      */
     abstract public function getDefaultDriver();
 

tests/Integration/Support/Fixtures/NullableManager.php

@@ -9,7 +9,7 @@ class NullableManager extends Manager
     /**
      * Get the default driver name.
      *
-     * @return string
+     * @return string|null
      */
     public function getDefaultDriver()
     {