[FIX] website: allow to get current website without fallback

Before this commit, when we ask for get_current_website, we always
get one website. If we don't know which, we took the first one.

Now we can ask, if we are into a website context or not with a new argument fallback.

Previous code like:
   website = get_current_website()
   if website:
       ...
Have now a sense ;)