fix: notice on empty buffer to flush in StreamTransport (#2)

```
[Application] Mar 21 23:56:01 |CRITICA| REQUES Uncaught PHP Exception ErrorException: "Notice: ob_flush(): Failed to flush buffer. No buffer to flush" at StreamTransport.php line 50 
```

Author: dazz

src/Server/Transport/Sse/StreamTransport.php

@@ -47,7 +47,9 @@ private function flushEvent(string $event, string $data): void
         echo sprintf('event: %s', $event).PHP_EOL;
         echo sprintf('data: %s', $data).PHP_EOL;
         echo PHP_EOL;
-        ob_flush();
+        if (ob_get_length() !== false) {
+            ob_flush();
+        }
         flush();
     }
 }