What should the term value in a tax_query for not filtering by that term?

I have a couple of dropdown’s that I’m using to filter the results for a custom query.
This is part of the code I use for filter the results.

// tax query
'tax_query' => array(
	'relation' => 'AND',
	array(
		'taxonomy' => 'zonas',
		'field' => 'slug',
		'terms' => $zona,
		'operator' => 'IN'
		)
	)

If an item of the dropDown “zona” was selected, that’s the value I use for the $zona variable.
What should be the value of $zona, if I don’t want to filter by that term? (when nothing is selected from the “zona” dropdown)
I tried with “NULL”, but that doesn’t work.
Thanks