Only show label element to shopp.quantity if set

Added missing str_true check to files to not show labels.

I also noticed a heavy amount of incosistencies with using if( !$string
) and Shopp:str_true($string) throughout the theme API's code - not that
it matters considerably.
Added !empty check for quantity label
Merge pull request #2837 from WazzaJB/master

Only show label element to shopp.quantity if set