nullable and searchable `MorphTo` displays 2nd input

- Laravel Version: 12.35.0
- Nova Version: v5.7.6

### Description:

`MorphTo` nullable field when it's `searchable` has 2nd input in the accessible state, even if them `null` option is chosen, than may confuse users:

```php

MorphTo::make('External Content Provider', 'externalContentProvider')
->types([
Article::class,
])
->withSubtitles()
->default(null)
->nullable()
->searchable()
->hideFromIndex()
```

Reproduce the shown nullable, searchable MorphTo configuration in Laravel Nova and inspect how the field renders after the null option is selected. Done means the second input is no longer accessible or displayed when the MorphTo value is null, without disrupting searchable non-null selections.

Written by the indexing model from the issue text.