Nil

From Shopify Wiki

Nil is a special "empty value" that is returned in certain circumstances by some Liquid properties and filters. It basically means "no result". It is not the same as the string of characters "nil". Nil is always treated as false in the conditions of {% if %} blocks and other Liquid tags that check for the truthfulness of a statement. For example, in a situation where a fulfillment does not yet have a tracking number entered, this if-block would never render the included text:

{% if fulfillment.tracking_number }}
We have a tracking number!
{% endif %}

Furthermore, nil will just not show up at all if you try to output it to the screen:

We don't have a tracking number, but here it is anyway: [{{ fulfillment.tracking_number }}]

Those square brackets around the nil tracking number would end up with nothing between them — no space, no nothing.