Merge branch '5.4' into 6.2

* 5.4:
  Avoid call on null
  [Notifier] Document Firebase options object in readme
  [ErrorHandler] Fix sending Vary header with SerializerErrorRenderer
  Fix intl data tests
  [DoctrineBridge] fix issue with missing stopwatch events
  [HttpClient] Fix canceling MockResponse

Author: nicolas-grekas

Tests/CurrenciesTest.php

@@ -736,7 +736,7 @@ public function testGetNumericCodeFailsIfNoNumericEquivalent($currency)
 
     public static function provideValidNumericCodes()
     {
-        $numericToAlpha3 = $this->getNumericToAlpha3Mapping();
+        $numericToAlpha3 = self::getNumericToAlpha3Mapping();
 
         return array_map(
             function ($numeric, $alpha3) { return [$numeric, $alpha3]; },
@@ -761,7 +761,7 @@ public function testForNumericCode($numeric, $expected)
 
     public static function provideInvalidNumericCodes()
     {
-        $validNumericCodes = array_keys($this->getNumericToAlpha3Mapping());
+        $validNumericCodes = array_keys(self::getNumericToAlpha3Mapping());
         $invalidNumericCodes = array_diff(range(0, 1000), $validNumericCodes);
 
         return array_map(
@@ -791,7 +791,7 @@ public function testExists()
         $this->assertFalse(Currencies::exists('XXX'));
     }
 
-    private function getNumericToAlpha3Mapping()
+    private static function getNumericToAlpha3Mapping()
     {
         $numericToAlpha3 = [];