FIX: Cannot unstake 0

[aakoshh]

Fixes an error discovered by consensus-shipyard/fendermint#319
Follow up to #257

The reason we can't allow 0 in unstake is because if we do, the next checkpoint fails:

checkpoint submission should succeed: CallError { exit_code: ExitCode { value: 33 }, failure_info: Some(MessageBacktrace(Backtrace { frames: [Frame { source: 64, method: 6, code: ExitCode { value: 33 }, message: "contract reverted" }, Frame { source: 64, method: 3844450837, code: ExitCode { value: 33 }, message: "contract reverted" }, Frame { source: 120, method: 6, code: ExitCode { value: 33 }, message: "contract reverted" }, Frame { source: 120, method: 3844450837, code: ExitCode { value: 33 }, message: "contract reverted" }], cause: None })), error: fendermint_vm_actor_interface::ipc::subnet::SubnetActorErrors:GatewayManagerFacetErrors(CannotReleaseZero(CannotReleaseZero)) }

[adlrocha]

Really good catch, the gateway prevents zero releases to prevent useless calls. Unfortunately, with the new staking module, this call has been hidden deep into the code :) Thanks!