Fix return type of grabHttpHeader method

Author: Naktibalda

src/Codeception/Module/REST.php

@@ -310,7 +310,7 @@ public function seeHttpHeaderOnce(string $name): void
      * @part json
      * @part xml
      */
-    public function grabHttpHeader(string $name, bool $first = true): string|array
+    public function grabHttpHeader(string $name, bool $first = true): string|array|null
     {
         return $this->getRunningClient()->getInternalResponse()->getHeader($name, $first);
     }

tests/unit/Codeception/Module/RestTest.php

@@ -384,6 +384,7 @@ public function testSeeHeaders()
         $this->assertSame('en-US', $this->module->grabHttpHeader('Content-Language'));
         $this->assertSame('no-cache', $this->module->grabHttpHeader('Cache-Control'));
         $this->assertSame(['no-cache', 'no-store'], $this->module->grabHttpHeader('Cache-Control', false));
+        $this->assertSame(null, $this->module->grabHttpHeader('Foo-Bar'));
     }
 
     public function testSeeHeadersOnce()