The HPR database is a bit short of NULL values.
The NULL value is meant to be used when there is no value. Usually, setting the 'NOT NULL' attribute means that a value must be provided, though a default could be defined if nulls are inappropriate. However, there are many cases where NULL, meaning "don't know", "TBA" or "not appropriate", could be useful.
Plus it's a lot easier and more efficient to write:
select count(*) from eps where tags is null;
select count(*) from eps where length(tags) = 0;
A query optimiser can do more clever things with the former.