chrisgee,
I might be misunderstanding your question, but when you add a list item to a Sharepoint list, the items will include all the columns. As such, if the user choses not to enter anything into a column, and that column isn't mandatory, then the list will indeed end up containing a blank field. You can't get Sharepoint to only display fields that aren't empty (well, not unless you implement something custom).
You could alter the validation rules on your InfoPath form, to prevent any user from not providing an entry in a given column - this would ensure that the Sharepoint column is always populated, but might not provide the functionality to meet your business requirements.
I guess I'd ask you why empty columns are causing you a problem in your Sharepoint list?
Hope this is in someway useful (and I haven't missed the point completely),
Regards,
Rob