stty: cannot set control chars to undefined with an empty string literal

[willshuttleworth]

passing an empty string literal for an stty mapping should set it to undefined

uutils coreutils, main branch

$ stty intr ''
thread 'main' panicked at src/uu/stty/src/stty.rs:614:14:
internal error: entered unreachable code: No arguments provided: must have been caught earlier
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

GNU coreutils stty:

$ stty intr ''
$ stty
speed 38400 baud; line = 0;
intr = <undef>;
-brkint -imaxbel iutf8