👻 stop allowing port 0

Allowing port 0 doesn't provide much values when doing testing, since user will need to run some command e.g. `netstat` to figure which port is assigned. And it also make defaulting port hard.

This change is not a breaking change since ad30658#diff-fbc18bb07cdd05391b7081acc1dfe170R81 has not been released in a stable release yet.

Author: Mengqi Yu

pkg/webhook/server.go

@@ -78,7 +78,7 @@ func (s *Server) setDefaults() {
 		s.WebhookMux = http.NewServeMux()
 	}
 
-	if s.Port < 0 {
+	if s.Port <= 0 {
 		s.Port = DefaultPort
 	}