[joy-ui][Autocomplete] Placeholder persists when values are selected in multiple mode

Steps to reproduce

1. Open the Joy UI Autocomplete docs: https://mui.com/joy-ui/react-autocomplete/#multiple
2. Add a placeholder prop to the multiple Autocomplete, e.g.:

<Autocomplete
  multiple
  placeholder="Select items"
  options={['Apple', 'Banana', 'Cherry']}
/>

3. Select one or more values

Current behavior

The placeholder text remains visible alongside the selected chips. The placeholder is passed unconditionally to the internal <input> element's additionalProps:

https://github.com/mui/material-ui/blob/da8d6e8b2b2c7f3898af475de3a4beb6c0eaf3ab/packages/mui-joy/src/Autocomplete/Autocomplete.tsx#L521

Since the input's value is always empty in multiple mode (selected values render as chips, not input text), the HTML placeholder never clears.

Expected behavior

The placeholder should be hidden when one or more values are selected, matching the behavior of Joy UI's Select component which was fixed in https://github.com/mui/material-ui/pull/39806.

Context

The fix in #39806 addressed this for Select but was not applied to Autocomplete.

Your environment

System:
  OS: macOS 26.2
Binaries:
  Node: 24.11.0
  npm: 11.6.1
Browsers:
  Chrome: 144.0.7559.133
  Safari: 26.2
npmPackages:
  @mui/joy: 5.0.0-beta.52
  @emotion/react: 11.14.0
  @emotion/styled: 11.14.1
  react: 17.0.2

Order ID: N/A (open source usage)

Search keywords: joy autocomplete placeholder multiple chips

Start in packages/mui-joy/src/Autocomplete/Autocomplete.tsx around the internal input additionalProps referenced in the issue, then compare the placeholder handling with the Select fix in pull request #39806. Verify the multiple Autocomplete reproduction with selected chips; done means the placeholder is hidden after one or more values are selected.