URLs that are created with stripes:link or stripes:url always contain the event name, even when none is given, so that the default handler will be used. I'd like to have a way to omit the default handler name, since most of the time, it doesn't have an @HandlesEvent annotation and the handler method name will be used.

UrlBuilder supports leaving off the event nameby accepting null as the event name. This is not possible with the link and url tags, since the event name is only set by LinkTagSupport when it's not-null.

I see two possible fixes (but maybe there are more elegant ones):
* allow null values for the event tag attribute (this requires checking whether event is null because it was never set or that it was explicitly set as null in buildUrl);
* treat an empty string as null for the event name.