Fix request.Timing

This is a temporary fix.

When working with page.on('request') the timing values haven't yet been
received, which are part of the response object. This will need to be
fixed later when we can wait for a response while working with
page.on('request').

internal/js/modules/k6/browser/common/http.go

@@ -361,6 +361,10 @@ type resourceTiming struct {
 
 // Timing returns the request timing information.
 func (r *Request) Timing() *resourceTiming {
+	if r.response == nil {
+		return nil
+	}
+
 	timing := r.response.timing
 
 	return &resourceTiming{