fix: type of return

The method getBody can't return null. The return of this method need to
be a instance of StreamInterface. To fix this I create a instance of a
class that extends StreamInterface.

Reference: https://github.com/php-fig/http-message/blob/1.0/src/MessageInterface.php#L169-L171

Signed-off-by: Vitor Mattos <vitor@php.rio>

src/HttpClient.php

@@ -5,6 +5,7 @@
 use ByJG\Util\Exception\CurlException;
 use ByJG\Util\Exception\NetworkException;
 use ByJG\Util\Exception\RequestException;
+use ByJG\Util\Psr7\NullStream;
 use InvalidArgumentException;
 use Psr\Http\Client\ClientInterface;
 use Psr\Http\Message\RequestInterface;
@@ -180,7 +181,7 @@ protected function setMethod(): void
     protected function setBody(): void
     {
         $stream = $this->request->getBody();
-        if (!is_null($stream)) {
+        if (!$stream instanceof NullStream) {
             if (!$this->getCurl(CURLOPT_POST) && !$this->getCurl(CURLOPT_CUSTOMREQUEST)) {
                 throw new RequestException($this->request,"Cannot set body with method GET");
             }

src/Psr7/Message.php

@@ -133,11 +133,12 @@ public function withoutHeader($name): MessageInterface
     /**
      * @inheritDoc
      */
-    public function getBody(): ?StreamInterface
+    public function getBody(): StreamInterface
     {
-        if (!is_null($this->body)) {
-            $this->body->rewind();
-        }
+        if (is_null($this->body)) {
+			$this->body = new NullStream();
+		}
+		$this->body->rewind();
         return $this->body;
     }
 
@@ -155,4 +156,4 @@ protected function normalize($header): string
     {
         return str_replace(" ", "-", ucwords(str_replace("-", " ", strtolower($header))));
     }
-}
+}