Prevents a file name from becoming blank when :restricted_characters => nil

[nuna]

cleanup_filename return nil when :restricted_characters is nil.

(I think that passing nil means `not restricted'.)

[sikachu]

Do we have a test case covering when :restricted_characters is nil? If not, do you mind adding the test, amend your commit, and force push to your branch?

Thank you.

[jyurek]

Even if all characters are restticted, the filename will remain _ and so won't be empty. Are you seeing other behavior? Also, we'll need a test case, as @sikachu said. Thanks.