add unit test for `CandidateEndorsement` methods

[saitofun]

What would you like to be added

Add unit test for CandidateEndorsement methods

in file action/candidate_endorsement.go

• [ ] Add tests to cover L28-29
• [ ] Add tests to cover L33-L34
• [ ] Add tests to cover L38-L39
• [ ] Add tests to cover L43-L45
• [ ] Add tests to cover L48-L49
• [ ] Add tests to cover L53-L56
• [ ] Add tests to cover L61-L63
• [ ] Add tests to cover L65-L67
• [ ] Add tests to cover L71-L80