fix assert_is_empty clippy lint

[xtqqczze]

$ cargo +beta clippy -q --workspace --all-targets --all-features
warning: used `assert!` to check that a value is not empty
  --> src/uucore/src/lib/features/encoding.rs:47:9
   |
47 |         assert!(!alphabet.is_empty());
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/beta/index.html#assert_is_empty
   = note: `-W clippy::assert-is-empty` implied by `-W clippy::pedantic`
   = help: to override `-W clippy::pedantic` add `#[allow(clippy::assert_is_empty)]`
help: use `assert_ne!` to show the value on failure
   |
47 -         assert!(!alphabet.is_empty());
47 +         assert_ne!(alphabet, []);
...