Yes, it is nullable and you can run update statement on these tables. There is system view called
sys.databases(Keeps attributes of database) which contains one row for each databases, you can't update it. If you do so, you will get error as below:
Msg 259, Level 16, State 1, Line 3
Ad hoc updates to system catalogs are not allowed.
So, only way to set recovery model is by running
alter database and its also mentioned on the link you have provided. These options are driven by drop-down at recovery model for each database and there you have only three options i.e.
Full, Simple and Bulk-logged.
This change can be done either by UI or through below command:
ALTER DATABASE DB_NAME SET RECOVERY SIMPLE -- You may change last keyword.
There is no blank option which could set the value as null in
recovery_model in table backupset can take only three values as mentioned at Official documentation:
Recovery model for the database:
I hope this answers your question.