I think you get it since you talk about time stamps. Just as you could record all mouse movements and keyboards to get higher fidelity you could break a single record into multiple with times stamps to record each step in a form, then the lack of record would be distinct from a record with all nulls along with time stamps. You could also do a record per field (which I have seen more than once) with metadata per field as other columns.

But without all that a system that supports null and some empty value gives you more fidelity than just null that again in my experience is practical.

Empty string and null work fine with strings for this purpose but for other datatypes you start needing to pick special values like the minimum value for a 32 bit int or 0 date etc.