[std.file] `rename` and `delete` with `null` parameters

https://github.com/dlang/phobos/pull/10980#issuecomment-4150523174

https://github.com/dlang/phobos/blob/master/std/file.d#L992-L993

https://github.com/dlang/phobos/blob/master/std/file.d#L1107-L1117