Psalm fixes

Signed-off-by: Aleksei Khudiakov <aleksey@xerkus.pro>

psalm-baseline.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<files psalm-version="5.25.0@01a8eb06b9e9cc6cfb6a320bf9fb14331919d505">
+<files psalm-version="5.26.0@4787eaf414e16c661902b94dfe5d882223e5b513">
   <file src="src/AbstractSerializer.php">
     <RedundantCondition>
       <code><![CDATA[! $crFound]]></code>
@@ -190,9 +190,6 @@
       <code><![CDATA[$headers]]></code>
       <code><![CDATA[$server]]></code>
     </MixedArgumentTypeCoercion>
-    <RedundantConditionGivenDocblockType>
-      <code><![CDATA[is_callable(self::$apacheRequestHeaders)]]></code>
-    </RedundantConditionGivenDocblockType>
   </file>
   <file src="src/ServerRequestFilter/FilterUsingXForwardedHeaders.php">
     <ImpureMethodCall>

src/ServerRequestFactory.php

@@ -20,7 +20,7 @@ class ServerRequestFactory implements ServerRequestFactoryInterface
     /**
      * Function to use to get apache request headers; present only to simplify mocking.
      *
-     * @var callable
+     * @var callable|string
      */
     private static $apacheRequestHeaders = 'apache_request_headers';
 
@@ -35,11 +35,11 @@ class ServerRequestFactory implements ServerRequestFactoryInterface
      *
      * @see fromServer()
      *
-     * @param array $server $_SERVER superglobal
-     * @param array $query $_GET superglobal
-     * @param array $body $_POST superglobal
-     * @param array $cookies $_COOKIE superglobal
-     * @param array $files $_FILES superglobal
+     * @param null|array $server $_SERVER superglobal
+     * @param null|array $query $_GET superglobal
+     * @param null|array $body $_POST superglobal
+     * @param null|array $cookies $_COOKIE superglobal
+     * @param null|array $files $_FILES superglobal
      * @param null|FilterServerRequestInterface $requestFilter If present, the
      *     generated request will be passed to this instance and the result
      *     returned by this method. When not present, a default instance of