fix(kv_typed): deser empty bytes with get_ref_slice

tools/kv_typed/src/as_bytes.rs

tools/kv_typed/src/backend/leveldb.rs

tools/kv_typed/src/backend/lmdb.rs

tools/kv_typed/src/backend/memory.rs

tools/kv_typed/src/backend/memory_singleton.rs

tools/kv_typed/src/backend/sled.rs