add unit tests for `action/protocol/staking/ethabi/stake_composite_bucketsbycandidate.go`

[saitofun]

What would you like to be added

add unit tests for action/protocol/staking/ethabi/stake_composite_bucketsbycandidate.go

• Add unit tests to cover L137
• Add unit tests to cover L141
• Add unit tests to cover L144
• Add unit tests to cover L180-L182
• Add unit tests to cover L186