Avoid panic when byte_pair_encode is given an empty piece

[devYRPauli]

byte_pair_encode special-cases a piece of length one but not length zero. An empty slice falls through to the piece_len < 100 branch into _byte_pair_merge, where for i in 0..piece.len() - 1 underflows usize and the following piece[i..i + 2] slice index panics.

This is reachable from Python:

>>> enc = tiktoken.get_encoding("cl100k_base")
>>> enc.encode("")
[]
>>> enc._encode_single_piece(b"")
pyo3_runtime.PanicException: range end index 2 out of range for slice of length 0

encode("") already returns an empty list, so the panic is an inconsistency rather than intended behaviour. byte_pair_split handles the same situation explicitly with assert!(piece.len() > 1).

The fix returns an empty vector for an empty piece, before the existing length-one check.

Added a regression test alongside the existing ones in src/lib.rs. Without the guard it panics at src/lib.rs:149; with it, cargo test passes 3/3.

Note: #559 proposed the same one-line guard. It was closed by its author without any maintainer response, so I do not believe the change was ever considered and rejected. Happy to close this if that is not the case.