Issue: You would like to have a blank/null option in the drop down. But sharepoint won't take SPACE as a valid option. You could specify the field to "not required" and initially it will be blank. But once some value has been specified there is no way to go back to blank. So?
Solution: Use ASCII code 0129. It is an empty character that SharePoint will accept as a valid option but it appears empty on the drop down. To enter the character, hold down the ALT key and type the number 0129.