Add nullable dependencies

src/Messaging/Frame.php

@@ -73,7 +73,7 @@ class Frame implements FrameInterface {
      * @param int         $opcode
      * @param callable<\UnderflowException> $ufExceptionFactory
      */
-    public function __construct($payload = null, $final = true, $opcode = 1, callable $ufExceptionFactory = null) {
+    public function __construct($payload = null, $final = true, $opcode = 1, ?callable $ufExceptionFactory = null) {
         $this->ufeg = $ufExceptionFactory ?: static function($msg = '') {
             return new \UnderflowException($msg);
         };

src/Messaging/MessageBuffer.php

@@ -1,7 +1,7 @@
 <?php
 namespace mfmdevsystem\RFC6455\Messaging;
 
-use Ratchet\RFC6455\Handshake\PermessageDeflateOptions;
+use mfmdevsystem\RFC6455\Handshake\PermessageDeflateOptions;
 
 class MessageBuffer {
     /**
@@ -82,13 +82,13 @@ class MessageBuffer {
     function __construct(
         CloseFrameChecker $frameChecker,
         callable $onMessage,
-        callable $onControl = null,
+        ?callable $onControl = null,
         $expectMask = true,
         $exceptionFactory = null,
         $maxMessagePayloadSize = null, // null for default - zero for no limit
         $maxFramePayloadSize = null,   // null for default - zero for no limit
-        callable $sender = null,
+        ?callable $sender = null,
-        PermessageDeflateOptions $permessageDeflateOptions = null
+        ?PermessageDeflateOptions $permessageDeflateOptions = null
     ) {
         $this->closeFrameChecker = $frameChecker;
         $this->checkForMask = (bool)$expectMask;