Add default empty string value for fields in MergeAuthzCollections command

SERVER-51866 introduced a small bug where the default empty string value for the `tempUsersCollection` and `tempRolesCollection` fields is not recognized, making both fields required. So if a mongorestore user wants to restore only the database users or only the database roles, they'll get the error described in TOOLS-2946.

Even though the server bug is fixed in SERVER-59515, we should make a redundant fix to maximize coverage for users with non-backported 4.9.0+ server binaries. We just need to add the default empty string value to the arguments if either the `users` or `roles` intents are null in (*MongoRestore).RestoreUsersOrRoles, i.e.

bson.E{

    Key:   "tempUsersCollection", // and "tempRolesCollection" separately

    Value: "",

}