Fix implicitly nullable parameters

Author: gsteel

src/Client.php

@@ -77,8 +77,8 @@ class Client implements HttpClient, HttpAsyncClient
      * @since 2.0 Accepts PSR-17 factories instead of HTTPlug ones.
      */
     public function __construct(
-        ResponseFactoryInterface $responseFactory = null,
-        StreamFactoryInterface $streamFactory = null,
+        ?ResponseFactoryInterface $responseFactory = null,
+        ?StreamFactoryInterface $streamFactory = null,
         array $options = []
     ) {
         $this->responseFactory = $responseFactory ?: Psr17FactoryDiscovery::findResponseFactory();

src/CurlPromise.php

@@ -51,15 +51,15 @@ public function __construct(PromiseCore $core, MultiRunner $runner)
      * If you do not care about one of the cases, you can set the corresponding callable to null
      * The callback will be called when the response or exception arrived and never more than once.
      *
-     * @param callable $onFulfilled Called when a response will be available
-     * @param callable $onRejected  Called when an error happens.
+     * @param callable|null $onFulfilled Called when a response will be available
+     * @param callable|null $onRejected  Called when an error happens.
      *
      * You must always return the Response in the interface or throw an Exception
      *
      * @return Promise Always returns a new promise which is resolved with value of the executed
      *                 callback (onFulfilled / onRejected)
      */
-    public function then(callable $onFulfilled = null, callable $onRejected = null)
+    public function then(?callable $onFulfilled = null, ?callable $onRejected = null)
     {
         if ($onFulfilled) {
             $this->core->addOnFulfilled($onFulfilled);

src/MultiRunner.php

@@ -81,7 +81,7 @@ public function remove(PromiseCore $core): void
      *
      * @param PromiseCore|null $targetCore
      */
-    public function wait(PromiseCore $targetCore = null): void
+    public function wait(?PromiseCore $targetCore = null): void
     {
         do {
             $status = curl_multi_exec($this->multiHandle, $active);