If the user entered an invalid quantity, Angular set the model to
undefined and we removed the input field to show the add button. That
makes it impossible for a user to see what the maximum quantity to enter
would be. For example:
- The variant has a stock level of 5.
- The user enters 7.
- Angular sets it to undefined.
- The input field disappears.
- The user is startled and doesn't know how to proceed.
But now we hide the input only if it's deliberately set to zero.
35b4e8c4d1