FIxes #9150 - Use default HttpExchange#setStreams implementation

Right now the JettyHttpExchangeDelegate#setStremas implementation just sets given parameters to respective steams. However, the default JDK implementation of HttpExchange#setStreams calls #ExchangeImpl.setStreams which in turn
only changes the respective streams when given parameters aren't null.
To follow this convention the  JettyHttpExchangeDelegate#setStremas was
rewritten to follow the same convention

jetty-http-spi/src/main/java/org/eclipse/jetty/http/spi/JettyHttpExchangeDelegate.java

@@ -209,7 +209,10 @@ public class JettyHttpExchangeDelegate extends HttpExchange
     @Override
     public void setStreams(InputStream i, OutputStream o)
     {
+        assert _is != null;
+        if (i != null)
             _is = i;
+        if (o != null)
             _os = o;
     }