fix: incorrect isDirty check and unit test

Author: jcjp

packages/vuetify/src/components/VCombobox/VCombobox.tsx

@@ -477,8 +477,7 @@ export const VCombobox = genericComponent<new <
         slots['append-item'] ||
         slots['no-data']
       )
-      const isEmptyString = model.value.length === 1 && model.value[0].value === ''
-      const isDirty = model.value.length > 0 && !isEmptyString
+      const isDirty = model.value.length > 0
       const textFieldProps = VTextField.filterProps(props)
 
       return (

packages/vuetify/src/components/VCombobox/__tests__/VCombobox.spec.browser.tsx

@@ -382,19 +382,22 @@ describe('VCombobox', () => {
     })
 
     it('should show placeholder if initial value is empty string', async () => {
-      render(() => (
+      const emptyString = ref('')
+
+      const { getByPlaceholderText } = render(() => (
         <VCombobox
-          v-model={''}
+          v-model={ emptyString.value }
           items={['a', 'b', 'c']}
           multiple
           itemTitle="text"
           itemValue="value"
+          placeholder="placeholder"
           returnObject
         />
       ))
 
-      const inputField = screen.getByCSS('.v-field')
-      expect(inputField).toBeUndefined()
+      const inputField = getByPlaceholderText("placeholder")
+      expect(inputField).toBeDisplayed()
     })
   })