Michel,
What client platform are you using? Special value is buggy when using a blank (empty) value. You are approaching it correctly by setting the initial value on the transaction property. Set a special value there of " " (blank space) with a display value of "-NONE-" on the complex table property. Then set the initial value to a constant of " " (blank space). Now the screen field should default to "-NONE-". You may need to remove the blank space on the backend processing code and set it back to an empty string before posting.
Jason Latko - Senior Product Developer at SAP