Allow button calls without a button name

* pulled the default value up from the DSL impl to the dsl - the default value has been there for a long time, but it was useless as it failed already on the DSL level
* differs somewhat from shoes3, which shows a big button with no text. I like this better.
* Only a PR because.. it is a bit questionable. E.g. the spec - I don't want to assert the name of the button as that might change and is not important to me here.

note: first person to say ok to this can gladly merge (if you ask me) - don't forget the Changelog though! :)