Resolve "Improve subscription logging"

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Related to #387

controller/nucleus/networkElementWatcher.go

@@ -200,6 +200,8 @@ func (n *NetworkElementWatcher) StopAndRemoveNetworkElementSubscription(subID uu
 // handleSubscribeResponse takes the subscribe response and additional information about the network element to distinguish
 // from which network element a subscribe response was sent including improved error handling.
 func (n *NetworkElementWatcher) handleSubscribeResponse(subscriptionInfo *transport.SubscriptionInformation, workerName string) {
+	log.Debugf("Received Subscribe response: MNE ID: %s, MNE Name: %s, SubResponse: %v", subscriptionInfo.NetworkElementID, subscriptionInfo.NetworkElementName, subscriptionInfo.SubResponse)
+
 	if subscriptionInfo.SubResponse == nil {
 		// Note: This needs proper error handling, no idea how yet. Simply logging would lead to spam in the console
 		// if the target that was subscribed to is not reachable anymore.
@@ -232,6 +234,11 @@ func (n *NetworkElementWatcher) handleSubscribeResponse(subscriptionInfo *transp
 func (n *NetworkElementWatcher) handleSubscribeResponseUpdate(resp *gpb.SubscribeResponse_Update, subscriptionInfo *transport.SubscriptionInformation) {
 	pathsAndValues := make(map[string]string, len(resp.Update.Update))
 
+	if resp.Update == nil || len(resp.Update.Update) == 0 {
+		log.Debugf("handleSubscribeResponseUpdate empty update or updates; Update: %v, InnerUpdates: %v", resp.Update, resp.Update.Update)
+		return
+	}
+
 	for _, update := range resp.Update.Update {
 		pathString, err := ygot.PathToString(update.Path)
 		if err != nil {