* Update lists.nim
* Update tlists.nim
* removed check `if b.tail != nil`
The tail of the list being null it is still possible to retrieve its end by going through all nodes from the head. So checking for null from `b.tail` is unnecessary. However, setting `a.tail = b.tail` only if `a.head != nil`, so you don't break a good list with an already broken one.
(cherry picked from commit dc5c88ca79)
afa4bc34b4