Trigger Logic (from SQL 6.5 )
I'm not sure if this code is fully accurate for columns that have more than 1 row with the same value, but I suppose that's a different subject.  What I see is that the only value the null counter is ever initialized to is 0.  That means it will never have an effect on the addition.  Putting on my dev hat, I could see the start of an unfinished concept.

If I needed to update a column to NULL, the where clause on line 9 wouldn't work.  The test NULL = NULL is false.  That means the join logic fails for update values of null.  That being said, it appears the error handling for that was never implemented beyond having a count column to use.