[BUG]: client may send empty key to cluster

[lxl66566]

currently xline client will not check if key is empty in requests like range() and delete() in crates/xline-client/src/clients/kv.rs, while etcd will check if key is empty before sending request.

although the with_prefix(), with_from_key() option could deal with empty key by push(0), but using with_range_end() will set a range_end without checking or modifying empty key. That could be a risk.

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[github-actions]

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