Feature request: QInput attribute nullable which returns null on empty string

[adam-iris]

This is sort of a followup to #17359 / #17895.

I wanted to more explicitly frame this as a feature request:

• This would be an extremely useful feature for QInput
• Given that this has been discussed for 8 years now in the Vue project, I'm skeptical of it being addressed there anytime soon

I am envisioning this as just an attribute that can be set on the component, eg.

<q-input
    type="number"
    v-model="value"
    nullable
/>