cli: Relax stake split destination check

[joncinque]

This is a copy of solana-labs#32735

Problem

In the CLI, process_stake_split checks that the destination stake account doesn't exist, and errors if it does. In most cases, this saves from getting an on-chain error from trying to split into some other account.

https://github.com/solana-labs/solana/blob/849525735f784f8e2e0fca19ba5699aea6b1724e/cli/src/stake.rs#L1884-L1893

However, it's possible to pre-seed some lamports into a system account before splitting into it, to keep the full amount delegated, so this check is needlessly strict.

Proposed Solution

Relax the check by allowing the destination split account to be owned by the system program, with some lamports, and no data. It can error in every other case still.

[CriesofCarrots]

Closed by #162