Address line blank in pdf letter
The issue isn't 100% clear because your formatting doesn't allow me to perfectly understand which tokens have a line break after them and which have a space - but I'm pretty sure I know what's going on.
By default, if a token evaluates to NULL and there is NOTHING else on that line, the line will be omitted entirely when producing address labels.

However, from your token listing, it seems that you have a space after {contact.supplemental_address_1}.  That's going to cause a space on the line, even if there's no supplemental address value.

If for formatting reasons you need a space there if there IS a supplemental address, there's a trick.  Let's look at the default mailing address format:

{contact.addressee}
{contact.current_employer}
{contact.street_address}
{contact.supplemental_address_1}
{contact.supplemental_address_2}
{contact.city}{, }{contact.state_province}{ }{contact.postal_code}


Note the {, } after the city and the { } after the state/province.  It's not well-documented, but that means, "include a ', ' after the city if a city exists" and "include a space after the state/province if a state/province exists".