Merge branch '5.4' into 6.2

* 5.4:
  [Translation][Mailer] Convert `$this` calls to static ones in data providers
  [BC Break] Make data providers for abstract test cases static
  use TestCase suffix for abstract tests in Tests directories
  Fix Request locale property doc types
  Bump absolute lowest dep to 4.4

Author: nicolas-grekas

Request.php

@@ -189,7 +189,7 @@ class Request
     protected $session;
 
     /**
-     * @var string
+     * @var string|null
      */
     protected $locale;
 
@@ -1367,7 +1367,7 @@ public function setLocale(string $locale)
      */
     public function getLocale(): string
     {
-        return null === $this->locale ? $this->defaultLocale : $this->locale;
+        return $this->locale ?? $this->defaultLocale;
     }
 
     /**