Is there a way for a missing numeric value to exist?

[CptAwe]

Columns which contain numbers and also have "NULL" or are empty, are replaced with zeroes. Is there a way to replace those missing values with a more unique value?

What if a missing value means something different than a zero value?

That already happens in columns that contain strings, where "NULL" or empty cells are replaced by null (which I might add, am very grateful for!).