Set empty list if preprocess_steps is None

[marukaz]

Fix #778

I considered three options to solve the problem:

1. reinstate None check
2. Change default of preprocess_steps to []
3. Assign [] in post init process if it's None

I chose 3 because

• if we do 1, same check will be needed whenever we add code for preprocess steps
• Regarding 2, as is well known, Python's default arguments are static and shared by all instances. Although this problem does not often cause confusion with Card, I would like to avoid it just in case

[marukaz]

I tried to solve src/unitxt/standard.py:189:9: C901 prepare is too complex (11 > 10) but I felt the code is reasonable, so added this file to ignore list.