clippy: fix if_then_some_else_none lint

[xtqqczze]

https://rust-lang.github.io/rust-clippy/master/index.html#if_then_some_else_none

[sunfishcode]

I have acquired a preference for avoiding .then in these kinds of functions in rustix. It makes the control flow a little harder to follow, and doesn't make the code that much easier to read.