As mentioned in the main article, there is an option called "Export empty metafields":

This option comes really handy in multiple cases, including (but not limited to):

1) In case you're trying to find out what products are missing a specific metafield.

2) In case you'd like to have a template file for data import generated automatically. By enabling this option and entering the namespace and key of a metafield that you'd like to populate with import later, you can generate a template file having all the system fields (product ID, metafield namespace, metafield key etc) populated for you

In order to activate this option, you have to:

1) specify the namespace of a metafield you'd like to have exported;

2) specify the key of a metafield you'd like to have exported;

3) tick the "Export empty metafields" box;

Here is a screenshot showing what a configured export would look like:

And here is a screenshot showing the export file: