Allow ott=null to mean "don't go anywhere" in a tour

[lentinj]

Currently we have "ott=null" ==> "Go to ~start". We'd like:

• [x] "ott=0" (or possibly -1) as "Go to ~start" instead
• [x] "ott=null" as don't go anywhere.

This will be especially useful in https://github.com/OneZoom/OZtree/issues/550