fix: (http/conn) When the body is empty, json_decode will throw an error.

Author: cclilshy

src/Net/Http/Server/Connection.php

@@ -521,7 +521,7 @@ private function parseRequestBody(): void
     {
         if ($this->method->value === 'POST') {
             if (str_contains($this->contentType, 'application/json')) {
-                $this->post = array_merge($this->post, json_decode($this->content, true) ?? []);
+                $this->post = array_merge($this->post, json_decode($this->content ?? '', true) ?? []);
             } elseif ($this->contentType === 'application/x-www-form-urlencoded') {
                 parse_str($this->content, $this->post);
             }