[3.12] gh-112087: Fix reduce logic for the empty reverse iterator for list

[corona10]

• Issue: Make list objects thread-safe in --disable-gil builds #112087

[corona10]

/python.exe -m test test_iter -R 3:3
Using random seed: 1549591648
Raised RLIMIT_NOFILE: 256 -> 1024
0:00:00 load avg: 8.69 Run 1 test sequentially
0:00:00 load avg: 8.69 [1/1] test_iter
beginning 6 repetitions
123456
......

== Tests result: SUCCESS ==