I don't know what to tell you. If you set the field to an empty string, then DMZ has to assume you want to save it. An empty string is not a NULL, and when inserting new data, DMZ only excludes NULLs from the query.
The best solution is to not set fields you want to be NULL to an empty string.
Also, if a field is EMPTY (not necessarily NULL), but not required, the validation routine will not run for that field. There is a workaround for this in the latest DMZ: the always_validate rule. It works like required, but doesn't through an error on empty strings.
You could add a rule that sets the default value if the string is empty, and then add the always_validate rule to ensure it runs. (Or you could add a rule that sets the value to NULL if empty, but the numeric rule will probably barf on NULL, so you'd have to override that one as well.)
