I assume you're creating fields on-the-fly (that don't already exist on the form) when creating new records with this code? (Since you mention the fields "not [getting] created", as opposed to problems inserting data into existing fields).
Can you post the code you're using? We ran some simple tests, and we were able to create Notes fields using the Newfield and AppendItemValue methods, even when the values were NULL. The fact that you're pulling these NULL values from a back-end data source shouldn't change things.
Notes doesn't understand NULLs, but what it does is to replace them with empty character strings.
This is documented here (note that LEI and LSXLC use the same underlying fundamentals, so this technote is still valid even though it specifies "LEI"):
http://www-01.ibm.com/support/docview.wss?uid=swg27003476
I'd like to see what your code is doing.