fix nullable prop for Vue

Author: RobinMalfait

packages/@headlessui-vue/src/components/combobox/combobox.test.ts

@@ -3476,7 +3476,7 @@ describe('Keyboard interactions', () => {
           let handleChange = jest.fn()
           renderTemplate({
             template: html`
-              <Combobox v-model="value">
+              <Combobox v-model="value" nullable>
                 <ComboboxInput />
                 <ComboboxButton>Trigger</ComboboxButton>
                 <ComboboxOptions>

packages/@headlessui-vue/src/components/combobox/combobox.ts

@@ -627,7 +627,7 @@ export let ComboboxInput = defineComponent({
         case Keys.Backspace:
         case Keys.Delete:
           if (api.mode.value !== ValueMode.Single) return
-          if (!api.nullable) return
+          if (!api.nullable.value) return
 
           let input = event.currentTarget as HTMLInputElement
           requestAnimationFrame(() => {