Set Psalm error level to 3; fix possibly null reason phrase in Response

psalm.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0"?>
 <psalm
-    errorLevel="4"
+    errorLevel="3"
     resolveFromConfigFile="true"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="https://getpsalm.org/schema/config"

src/Message/Response.php

@@ -103,7 +103,7 @@ class Response extends Message implements ResponseInterface
 
     /**
      * @param int $statusCode
-     * @return string|null
+     * @return string
      */
     private function getDefaultReasonPhraseForStatusCode($statusCode)
     {
@@ -167,9 +167,6 @@ class Response extends Message implements ResponseInterface
             510 => "Not Extended",
             511 => "Network Authentication Required"
         ];
-        if (isset($reasonPhraseLookup[$statusCode])) {
-            return $reasonPhraseLookup[$statusCode];
-        }
-        return null;
+        return $reasonPhraseLookup[$statusCode] ?? '';
     }
 }