[v13] Fix nil user group entries. (#29326)

* Fix nil user group entries.

When user groups are being looked up while listing applications if a user
group is missing from the lookup, the previous logic would inadvertently
leave nil user groups if the lookup failed. This has been corrected to ensure
that no nil entries end up in the user group lookup.

* Adjust unit test to exercise this.

lib/web/apiserver_test.go

@@ -4284,7 +4284,7 @@ func TestClusterAppsGet(t *testing.T) {
 				Spec: types.AppSpecV3{
 					URI:        "https://console.aws.amazon.com", // sets field awsConsole to true
 					PublicAddr: "publicaddrs",
-					UserGroups: []string{"ug1"},
+					UserGroups: []string{"ug1", "ug2"}, // ug2 doesn't exist in the backend, so its lookup will fail.
 				},
 			},
 		},

lib/web/apps.go

@@ -26,7 +26,6 @@ import (
 	"github.com/gravitational/trace"
 	"github.com/julienschmidt/httprouter"
 
-	"github.com/gravitational/teleport/api/client"
 	apiclient "github.com/gravitational/teleport/api/client"
 	"github.com/gravitational/teleport/api/client/proto"
 	apidefaults "github.com/gravitational/teleport/api/defaults"
@@ -60,7 +59,7 @@ func (h *Handler) clusterAppsGet(w http.ResponseWriter, r *http.Request, p httpr
 		return nil, trace.Wrap(err)
 	}
 
-	page, err := client.GetResourcePage[types.AppServer](r.Context(), clt, req)
+	page, err := apiclient.GetResourcePage[types.AppServer](r.Context(), clt, req)
 	if err != nil {
 		return nil, trace.Wrap(err)
 	}
@@ -86,15 +85,15 @@ func (h *Handler) clusterAppsGet(w http.ResponseWriter, r *http.Request, p httpr
 
 		app := server.GetApp()
 
-		ugs := make(types.UserGroups, len(app.GetUserGroups()))
-		for i, userGroupName := range app.GetUserGroups() {
+		ugs := types.UserGroups{}
+		for _, userGroupName := range app.GetUserGroups() {
 			userGroup := userGroupLookup[userGroupName]
 			if userGroup == nil {
 				h.log.Debugf("Unable to find user group %s when creating user groups, skipping", userGroupName)
 				continue
 			}
 
-			ugs[i] = userGroup
+			ugs = append(ugs, userGroup)
 		}
 		sort.Sort(ugs)
 		appsToUserGroups[app.GetName()] = ugs