fix(restore): Handle MaxUid=0 appropriately (#7258)

At the end of the restore operation we update the MaxUid in zero. This
MaxUid can be zero if the backup was created from an empty DB.

(cherry picked from commit b05fb93)

ee/backup/run.go

@@ -15,10 +15,11 @@ package backup
 import (
 	"context"
 	"fmt"
-	"google.golang.org/grpc/credentials"
 	"os"
 	"time"
 
+	"google.golang.org/grpc/credentials"
+
 	"github.com/dgraph-io/dgraph/ee/enc"
 	"github.com/dgraph-io/dgraph/protos/pb"
 	"github.com/dgraph-io/dgraph/worker"
@@ -236,19 +237,19 @@ func runRestoreCmd() error {
 		ctx, cancelTs := context.WithTimeout(context.Background(), time.Minute)
 		defer cancelTs()
 
-		_, err := zc.Timestamps(ctx, &pb.Num{Val: result.Version})
-		if err != nil {
+		if _, err := zc.Timestamps(ctx, &pb.Num{Val: result.Version}); err != nil {
 			fmt.Printf("Failed to assign timestamp %d in Zero: %v", result.Version, err)
 			return err
 		}
 
-		ctx, cancelUid := context.WithTimeout(context.Background(), time.Minute)
-		defer cancelUid()
-
-		_, err = zc.AssignUids(ctx, &pb.Num{Val: result.MaxLeaseUid})
-		if err != nil {
-			fmt.Printf("Failed to assign maxLeaseId %d in Zero: %v\n", result.MaxLeaseUid, err)
-			return err
+		// MaxLeaseUid can be zero if the backup was taken on an empty DB.
+		if result.MaxLeaseUid > 0 {
+			ctx, cancelUid := context.WithTimeout(context.Background(), time.Minute)
+			defer cancelUid()
+			if _, err = zc.AssignUids(ctx, &pb.Num{Val: result.MaxLeaseUid}); err != nil {
+				fmt.Printf("Failed to assign maxLeaseId %d in Zero: %v\n", result.MaxLeaseUid, err)
+				return err
+			}
 		}
 	}