Check for empty ns list before checking nslist[0]

In determining whether a backup includes all namespaces, item_collector
checks for an empty string in the first element of the ns list. If processing
includes+excludes results in an empty list, treat this as another case
of a not-all-namespaces backup rather than crashing velero.

Signed-off-by: Scott Seago <sseago@redhat.com>

changelogs/unreleased/5302-sseago

@@ -0,0 +1 @@
+Check for empty ns list before checking nslist[0]

pkg/backup/item_collector.go

@@ -225,8 +225,11 @@ func (r *itemCollector) getResourceItems(log logrus.FieldLogger, gv schema.Group
 
 	namespacesToList := getNamespacesToList(r.backupRequest.NamespaceIncludesExcludes)
 
-	// Check if we're backing up namespaces, and only certain ones
-	if gr == kuberesource.Namespaces && namespacesToList[0] != "" {
+	// Check if we're backing up namespaces for a less-than-full backup.
+	// We enter this block if resource is Namespaces and the namespae list is either empty or contains
+	// an explicit namespace list. (We skip this block if the list contains "" since that indicates
+	// a full-cluster backup
+	if gr == kuberesource.Namespaces && (len(namespacesToList) == 0 || namespacesToList[0] != "") {
 		resourceClient, err := r.dynamicFactory.ClientForGroupVersionResource(gv, resource, "")
 		if err != nil {
 			log.WithError(err).Error("Error getting dynamic client")