Combobox: fix primary icon visibility

The primary icon for a Combobox was never visible if a value was
selected because we were always attempting to use the icon for that item
even if it didn't have one.

src/lib/Combobox.svelte

@@ -654,9 +654,9 @@
 			>
 				{#if loading}
 					<Spinner class="stroke-sui-accent! -mt-0.5" size="1em" />
-				{:else if useHighlighted && highlighted}
+				{:else if useHighlighted && highlighted && highlighted.icon}
 					{@render optionIcon(highlighted)}
-				{:else if value}
+				{:else if value && value.icon}
 					{@render optionIcon(value)}
 				{:else if icon}
 					{#if typeof icon === 'function'}