mcs: fix panic of get leader (#7472)

close #7280

Signed-off-by: Ryan Leung <rleungx@gmail.com>

Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>

pkg/member/participant.go

@@ -121,22 +121,16 @@ func (m *Participant) Client() *clientv3.Client {
 // IsLeader returns whether the participant is the leader or not by checking its leadership's
 // lease and leader info.
 func (m *Participant) IsLeader() bool {
-	if m.GetLeader() == nil {
-		return false
-	}
 	return m.leadership.Check() && m.GetLeader().GetId() == m.member.GetId() && m.campaignCheck()
 }
 
 // IsLeaderElected returns true if the leader exists; otherwise false
 func (m *Participant) IsLeaderElected() bool {
-	return m.GetLeader() != nil
+	return m.GetLeader().GetId() != 0
 }
 
 // GetLeaderListenUrls returns current leader's listen urls
 func (m *Participant) GetLeaderListenUrls() []string {
-	if m.GetLeader() == nil {
-		return nil
-	}
 	return m.GetLeader().GetListenUrls()
 }
 
@@ -149,13 +143,9 @@ func (m *Participant) GetLeaderID() uint64 {
 func (m *Participant) GetLeader() participant {
 	leader := m.leader.Load()
 	if leader == nil {
-		return nil
-	}
-	member := leader.(participant)
-	if member.GetId() == 0 {
-		return nil
+		return NewParticipantByService(m.serviceName)
 	}
-	return member
+	return leader.(participant)
 }
 
 // setLeader sets the member's leader.