nullable and searchable `MorphTo` displays 2nd input

[alies-dev]

• 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:

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