I use the following query to get a good overview of running Backups / LOG Shipping Status.
SELECT [b].[database_name] ,
script='RESTORE LOG DBNAME FROM DISK=N''' + f.physical_device_name + ''' WITH NORECOVERY; '
FROM [msdb].[dbo].[backupset] AS [b]
LEFT JOIN msdb.dbo.backupmediafamily f ON b.media_set_id = f.media_set_id
WHERE [b].[database_name] = 'DBNAME'
AND [b].[backup_start_date] >= '2017-05-10 13:00:00.000' ORDER BY [b].[backup_start_date] asc;
It gives a picture of the Backups taken and so you will See which Backups you need to RESTORE them on secondary DB.
The Backups Marked yellow need to be restored on 2ndary, only those.
Regarding your NUL backup, have you already scripted All of the stored procedures in Place and Looked for NUL in there?
Have you observed any Logins the time this NUL Backup happens? If a login FROM outside starts this Backup you could cut the login off the Server.
Hope it helps.