Task not completable when it contains a step with reflex enabled but no target element

A reflex step must always have a target element, otherwise the user has nothing to click to advance to the next step. This will block the task, with no way forward. There should be a check for this situation, to render these reflex+no targetElement steps as if they were non-reflex steps.

There could also be a warning for people editing the task when there is a step which has `reflex`=Yes and an empty `targeted element`, to make sure these kinds of issues are noticed.

Original comment: https://github.com/xwiki-contrib/guided-tour/pull/53#discussion_r4313932127