Recovery_model for backupset table null sqlserver

Going through https://www.sqlshack.com/understanding-database-recovery-models/ I see that there are three types of recovery models: - SIMPLE - FULL - BULK_Logged But for the table msdb.dbo.backupset I can see the recovery_model column can be null. In what case would this take a NULL value. How to create a backup such that it falls into this case.