Prevent empty request body stream in HttplugClient and Psr18Client

This prevents adding `Content-Length` and `Transfer-Encoding` headers
when sending a request with an empty body using CurlHttpClient

Author: ajgarlag

HttplugClient.php

@@ -227,11 +227,13 @@ private function sendPsr7Request(RequestInterface $request, ?bool $buffer = null
             $headers = $request->getHeaders();
 
             $size = $request->getHeader('content-length')[0] ?? -1;
-            if (0 > $size && 0 <= $size = $body->getSize() ?? -1) {
+            if (0 > $size && 0 < $size = $body->getSize() ?? -1) {
                 $headers['Content-Length'] = [$size];
             }
 
-            if (0 <= $size && $size < 1 << 21) {
+            if (0 === $size) {
+                $body = '';
+            } elseif (0 < $size && $size < 1 << 21) {
                 if ($body->isSeekable()) {
                     try {
                         $body->seek(0);

Psr18Client.php

@@ -91,11 +91,13 @@ public function sendRequest(RequestInterface $request): ResponseInterface
             $headers = $request->getHeaders();
 
             $size = $request->getHeader('content-length')[0] ?? -1;
-            if (0 > $size && 0 <= $size = $body->getSize() ?? -1) {
+            if (0 > $size && 0 < $size = $body->getSize() ?? -1) {
                 $headers['Content-Length'] = [$size];
             }
 
-            if (0 <= $size && $size < 1 << 21) {
+            if (0 === $size) {
+                $body = '';
+            } elseif (0 < $size && $size < 1 << 21) {
                 if ($body->isSeekable()) {
                     try {
                         $body->seek(0);