feat(tests): use strict assert

Author: Rotzbua

.php-cs-fixer.php

@@ -8,6 +8,7 @@
     'blank_line_after_opening_tag'        => false, // Do not waste space between <?php and declare.
     'global_namespace_import'             => ['import_classes' => false, 'import_constants' => false, 'import_functions' => false],
     'php_unit_test_class_requires_covers' => true,
+    'php_unit_construct'                  => true,
     'php_unit_method_casing'              => true,
     // Do not enable by default. These rules require review!! (but they are useful)
     // '@PHP80Migration:risky'      => true,

tests/SubscriptionTest.php

@@ -14,18 +14,18 @@ public function testCreateMinimal(): void
         ];
         $subscription = Subscription::create($subscriptionArray);
         $this->assertEquals("http://toto.com", $subscription->getEndpoint());
-        $this->assertEquals(null, $subscription->getPublicKey());
-        $this->assertEquals(null, $subscription->getAuthToken());
-        $this->assertEquals(null, $subscription->getContentEncoding());
+        $this->assertNull($subscription->getPublicKey());
+        $this->assertNull($subscription->getAuthToken());
+        $this->assertNull($subscription->getContentEncoding());
     }
 
     public function testConstructMinimal(): void
     {
         $subscription = new Subscription("http://toto.com");
         $this->assertEquals("http://toto.com", $subscription->getEndpoint());
-        $this->assertEquals(null, $subscription->getPublicKey());
-        $this->assertEquals(null, $subscription->getAuthToken());
-        $this->assertEquals(null, $subscription->getContentEncoding());
+        $this->assertNull($subscription->getPublicKey());
+        $this->assertNull($subscription->getAuthToken());
+        $this->assertNull($subscription->getContentEncoding());
     }
 
     public function testCreatePartial(): void