fix(NcListItem): change condition to display the counter number

[julien-nc]

The previous condition was always showing the counter number because the counter-number prop has 0 as default value.

This can be seen in a few examples in https://next--nextcloud-vue-components.netlify.app/#/Components/NcListItems?id=nclistitem

<NcListItem compact
	name="Name of the element">
	<template #icon>
		<IconNoteText :size="20" />
	</template>
</NcListItem>