Hello Miroslav,
Thank you for your post.
The issue you have faced is untypical, as the Store Manager does not populate a text fields with 'null'.
Please submit a ticket at:
http://support.emagicone.com/submit_ticket and specify your actions step-by-step. Also please attach the store configuration file*, so we could check your database more deeply.
In case you have imported product, please provide us with your the catalog file and import configuration file.