The straightforward reason for this request is that there's currently no way to create an optional numeric field using v-model since empty inputs are reset to zero.

The fuzzier reason is that in many if not most models an empty input signifies the absence of its value, hence the error message "Please enter a username" instead of "Please enter a username that isn't empty," and the fact that, in contrast to data validation tools like JSON schema, HTML5 doesn't allow required fields to be empty.

I have no idea what this modifier should be called, though.