fill_form sending empty selectors to set_value causes timeouts

[spockNinja]

I think somewhere in 5092aa8, a change was introduced that uses set_value in fill_form, but it sends an empty selector. Then when set_value calls wait_for_visible(selector, **kwargs), it times out.

No rush here, as I've updated BC to not use fill_form, but I'm sure someone else will run into it eventually.