How to Stop OleDbDataReader Getting NULL(s)

Question

Hi guys ,
I am doing a very simple connection with C# and Ms Access and so far every thing goes fine, with me!.I just have a problem on retrieving Null values.the code that I am using to list values of a field in a table is like:

OleDbCommand cmd = new OleDbCommand(sql, cn);
reader = cmd.ExecuteReader();
while (reader.Read()){
listAtt.Items.Add(reader.GetValue(0).ToString());
}

as I told you this code works fine and get values of the field but it it return NULL values as well and when I want to list the values in a ListBox they appears like a space in the listbox display as they are a type of data.
Now my question is that how I can stop getting Nulls or not displaying them on the ListBox?!
I appriciate if you give me a good hint on that and also provide me some hints or #Ref to teach me how I can display the query results on a Grid format like a real table(Tabular) in a RichTextbox(or TextBox)?

Thanks again
Behrouz