Fix length for empty response to prevent chunked stream

The java HttpExchange expects length -1 to send Content-length: 0 ...

src/main/java/org/asamk/signal/http/HttpServerHandler.java

@@ -59,7 +59,7 @@ public class HttpServerHandler {
 
             httpExchange.getResponseBody().write(byteResponse);
         } else {
-            httpExchange.sendResponseHeaders(status, 0);
+            httpExchange.sendResponseHeaders(status, -1);
         }
 
         httpExchange.getResponseBody().close();