Description

The preview rule PLR6201 turns 1 in [] into 1 in {}. Now, that code is pretty useless and Ruff should probably complain about it somehow, but turning [] into {} is problematic for a couple of reasons:

• The rule says you should use a set, but {} is an empty dictionary, not an empty set.
• It's strictly more likely to throw an error, because unhashable_object in {} raises an error.
• It's slower (because it has to hash the object).

I'd recommend to not trigger PLR6201 on an empty collection, and possibly to add a separate lint rule that warns against in where the right-hand side is an empty collection.