Update list.py

[John-J-Riehl]

Found an error when slicing a ListMonad.

my_list = ListMonad(1)
print(my_list[:-1]) # should be [], actually raises UnboundLocalError on line 84 in list.py

I believe the change I made will fix this.

[maestro-1]

This is actually pretty accurate and I think it is a fix that should get merged. Although removing the else and just returning outside the if block is more aesthetically pleasing to look at @John-J-Riehl