My images paths contains NULL I have this content type called "Product" that contains a "Multimedia asset" field (Media module), called Images.
Everything is working (I can access the image), except all my upload images have "null" in their path,e.g. "mydomain.com/sites/all/files/null/product-928.jpg."

The same happens when the image is rendered with a image style; in that case the path is "mydomain.com/sites/all/files/styles/table_icon/public/null/product-928.jpg."

("table_icon" is the name of my image style.)

I am using Drupal 7.10 and the Media module 7.x-1.0-rc2.

Do you have any idea on how to solve this problem?

Thanks!
This is a known bug with the media module. See Issue #1351890: Wrong upload destination folder "null" and Issue #1364676: Upload path. The latter of the two contains a patch to fix this.