There is no setting to change this behavior. One suggestion; since this is a DataTable object, you could write a quick C# script to replace all of those values with something else. Essentially, iterate all rows and columns and add them to a new DataTable that your script returns. This table would then no longer contain the DBNull values.
Yes, writing a script to iterate over the table is a quick albeit inefficient way to achieve the same thing, but let's flog this horse a little bit more.
DataColumn has two properties: AllowDBNull and DefaultValue. I tried many combinations of setting this value within the DataTable of the LookupTable but it always errors out because the LookupTable is copying from the DataTable internally created by the Excel component ExportData. If there were a way of setting these two values within the Excel component it should solve the problem - is there a way to do this without development changing the underlying toolset? If not, is it something we can reasonably make a change request for?
The use case is not critical and is certainly not in any way urgent, but I suspect is quite pervasive and the development change should be quite easy.
You can certainly request a product change through your account contacts. This forum would not be the place to do that though as the relevant parties are not present. Unless the table is large, the iteration done via a script would be very quick (milliseconds I'd guess in my recent experience...certainly less than a second). It is inefficient though, no matter how much extra time it adds.