MM-9977: test config.SetDefaults leaves nothing nil (#8610)

* MM-9977: test config.SetDefaults leaves nothing nil

* clarify test default config test cases

* comment re: allowing nil slice

* extend config SetDefaults to handle partially initialized configs

model/config.go

@@ -1715,8 +1715,8 @@ func (s *MessageExportSettings) SetDefaults() {
 
 	if s.GlobalRelaySettings == nil {
 		s.GlobalRelaySettings = &GlobalRelayMessageExportSettings{}
-		s.GlobalRelaySettings.SetDefaults()
 	}
+	s.GlobalRelaySettings.SetDefaults()
 }
 
 type DisplaySettings struct {