Merge branch '5.4' into 6.0

* 5.4:
  [Mime] Fix invalid DKIM signature with multiple parts
  Fix Command::run phpdoc
  Update Response.php
  [HttpFoundation] Fix TypeError on null `$_SESSION`  in `NativeSessionStorage::save()`

Author: fabpot

Response.php

@@ -72,7 +72,7 @@ class Response
     public const HTTP_PRECONDITION_REQUIRED = 428;                                       // RFC6585
     public const HTTP_TOO_MANY_REQUESTS = 429;                                           // RFC6585
     public const HTTP_REQUEST_HEADER_FIELDS_TOO_LARGE = 431;                             // RFC6585
-    public const HTTP_UNAVAILABLE_FOR_LEGAL_REASONS = 451;
+    public const HTTP_UNAVAILABLE_FOR_LEGAL_REASONS = 451;                               // RFC7725
     public const HTTP_INTERNAL_SERVER_ERROR = 500;
     public const HTTP_NOT_IMPLEMENTED = 501;
     public const HTTP_BAD_GATEWAY = 502;

Session/Storage/NativeSessionStorage.php

@@ -223,7 +223,7 @@ public function save()
                 unset($_SESSION[$key]);
             }
         }
-        if ([$key = $this->metadataBag->getStorageKey()] === array_keys($_SESSION)) {
+        if ($_SESSION && [$key = $this->metadataBag->getStorageKey()] === array_keys($_SESSION)) {
             unset($_SESSION[$key]);
         }
 

Tests/Session/Storage/NativeSessionStorageTest.php

@@ -311,4 +311,13 @@ public function testRegenerateInvalidSessionIdForNativeFileSessionHandler()
         $this->assertTrue($started);
         $this->assertSame('&~[', session_id());
     }
+
+    public function testSaveHandlesNullSessionGracefully()
+    {
+        $storage = $this->getStorage();
+        $_SESSION = null;
+        $storage->save();
+
+        $this->addToAssertionCount(1);
+    }
 }