When I debug my MO application, I came across this issue that MapObjects returns NULL value as a zero. I was relating external .mdb table to shapefile and querying the records based on field from the table. The query passed to Search Expression returns the erroneaus recordset taking null values as zeros. I was puzzled to see the result, after through debugging I understood its MO problem. When I searched forum, I found detailed explanation on this issue from Mr. A.J. Romanelli . Below link helps you to understand the basic concept behind the problem.
In nutshell “The dBase specification (and thus shapefiles) doesn’t support null values for string fields, but null values are supported for numeric fields (numeric nulls are stored as null characters, not zero values). MO doesn’t support the dBase null value numeric, and returns the null values as zeros.”
We need to use some other workaround for this problem.