Is that inputText based on a Entity attribute?
It corresponds to a database table field of type number.
Well your answer was not clear.
But i'll assume that you created an Entity Object and a View Object ... ie: (Business Components) for that table.
If you didn't create business components for that table then you should create it to force validation on it.
To force validation on any fields in you table views you should always start with the Entity Object.
So please go to the Entity Object for that table ...
1- Choose "Business Rules" from the list on the left.
2- Choose the attribute you want to force validation on from the list on the right.
3- Click the little green plus sign in the header of the attributes list.
4- In the dialogue (Based on your case) choose:
Compare with: Literal value
And then in the Enter literal value box type 0.
5- Final step is to specify an error message so in the same dialogue choose the "Failure Handling" tab and type you message.
And that's it ... And please read this for better understanding: https://docs.oracle.com/cd/E17904_01/web.1111/b31974/bcvalidation.htm#ADFFD396