fix: do not allow nil Callbacks functions

If user specifies a nil callback, then error out
rather than crashing at runtime due to a nil reference.

staging/src/k8s.io/client-go/tools/leaderelection/leaderelection.go

@@ -89,6 +89,12 @@ func NewLeaderElector(lec LeaderElectionConfig) (*LeaderElector, error) {
 	if lec.RetryPeriod < 1 {
 		return nil, fmt.Errorf("retryPeriod must be greater than zero")
 	}
+	if lec.Callbacks.OnStartedLeading == nil {
+		return nil, fmt.Errorf("OnStartedLeading callback must not be nil")
+	}
+	if lec.Callbacks.OnStoppedLeading == nil {
+		return nil, fmt.Errorf("OnStoppedLeading callback must not be nil")
+	}
 
 	if lec.Lock == nil {
 		return nil, fmt.Errorf("Lock must not be nil.")