If you have really wide ranging items, perhaps a lookup tables makes sense.

Personally, I never would use NULL in the field as an update. If someone wanted to remove data from a field, I'd set it to 0, or '', or something else. I know I might run into issues, but so far that's worked.

For me, I've used NULL as the default value. That means the parameter was not passed, and I ignore it in code. If I needed to pass in NULL for that field, I might use another method, maybe a different parameter (bit) that says use the field as NULL.