You can either create one more scale point as - "None" and change custom validation accordingly.
OR
You can make matrix multi answer and check "Make answer exclusive" for all the scale points

Hi rondev ,
Thank you for your answer.
The multi answer option is not viable, because once you choose one choice, the remaining choices are automatically deselected, and I need the other answers.
Creating one more scale point doesn't make sense in my case, here it is one of the questions (this one is related to soft skills):
The only problem is that, if the participant chooses by mistake one of the 5 choices in the "Other, please specify" statement, he/she cannot deselect the choice, and for the survey to continue, the participant will have to write something in the text entry box, even if it is not important for the question itself.

Agreed, adding new column will make custom validation more complex. But check the below solution.
"The multi answer option is not viable, because once you choose one choice, the remaining choices are automatically deselected, and I need the other answers." - In single choice too, this happens. So we are just reflecting the functionality of single answer to multi answer, with added feature of deselecting choices once selected.

Thank you so much rondev
That worked, I was doing it wrong, when I tryed it before, I was putting the statements exclusive and not the scale points
I really appreciate your help!