Fix conditions in ActiveStorage

[santib]

Summary

I first wanted to fix this condition !attachables.nil? || Array(attachables).any? since if something is not nil it will always return true instead of checking if the array has any element. My guess is that this other condition that already existed attachables.nil? || Array(attachables).none? was tried to be negated unsuccessfully.

Then I realised that Array(nil) returns [] so we could simplify the conditions.

Notes

Not sure why tests are failing

[georgeclaghorn]

The original condition (attachables.nil? || Array(attachables).none?) was intended to avoid needless array allocations, but I think it’s clearer this way.