Filter blocks with dropdown display style should also show the chevron when Single selection is enabled

[Aljullu]

To reproduce

1. Create a post or page with the Filter by Stock, Filter by Rating and Products blocks.
2. Select the Filter by Stock Controls block and choose: Single and Dropdown in the Display Settings.
3. Select the Filter by Rating Controls block and choose: Multiple and Dropdown in the Display Settings.
4. Go to the frontend.
5. Notice the Filter by Stock dropdown doesn't have a chevron on the right, while the Filter by Rating block does have it.

Expected behavior

Both dropdowns should display the chevron to signal to the user that clicking on them will open a list of suggestions.

Additional context

The relevant code can be found here:

woocommerce-blocks/assets/js/blocks/attribute-filter/block.tsx

Lines 669 to 671 in 32d4605

	{ multiple && (
	<Icon icon={ chevronDown } size={ 30 } />
	) }