If there is missing data to occur within a column, then pragmatically, you split that column off into its own table with the associated key from the original and store only those keys that have valid values.
If you do anything else, such as using nulls, then you are just creating a problem that WILL come back and bite you in the nether regions of your psyche.
Of course, there will other opinions about how to handle this. As far as I am concerned, Nulls are a curse foisted on us by those vendors and standards bodies who took the the easy way out.
Demonstrate that it will bite you immediately. This technique has ensured that only valid data has been kept and that reporting works.
When nulls are stored there are no guarantees that anything you ask of the database will ever turn out right. Especially when there are millions of records stored. Two queries that should give you the same answer give different results when nulls exist. Seen it too often.
Table separation leads to record fragmentation and big IO cost on record fetching. Your ideal academic world will be crashed under production reality. "Millions of records". Bwa-ha-ha.
If your DBMS is so poorly written that record fragmentation is an issue then you need to change the DBMS. Since most (>99%) of the database design and implementation work that I have been involved since the mid-80's was business related and for a variety of different industries, I didn't find table separation to be a problem. The appropriate designs led to faster applications.
I have also worked for companies that didn't use relational database theory for their products and they had far more issues. In a couple, I was able to hive off the database designs from the main systems and got the applications to actually work and work properly.
If you do anything else, such as using nulls, then you are just creating a problem that WILL come back and bite you in the nether regions of your psyche.
Of course, there will other opinions about how to handle this. As far as I am concerned, Nulls are a curse foisted on us by those vendors and standards bodies who took the the easy way out.