Fix the find filter to return nil when filtering empty arrays

[karreiro]

This PR is a patch that addresses an issue in the find filter introduced by #1869.

When filtering an empty array, the find filter should return nil, just as it returns the found element when it locates the desired element or nil when the element cannot be found. However, it was incorrectly returning an empty array.

[frandiox]

Code looks good 👍 -- What about find_index, though?

[karreiro]

Thanks a lot for the review, @frandiox! I've applied the fixes for find_index and has as well :)