Refactor: cts: Drop 0-check from Environment.getitem()

This is a leftover from commit 514bffa2. At that time, the Environment
class did not have a contains() method. Python fell back to
getitem(0) if both contains() and iter() were undefined.

https://docs.python.org/3.8/reference/expressions.html#membership-test-details

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>