I’ve gone through and removed all duplicate occurrences of the various smarty function so that the standard version in the library is used. There is no need to include a function in a invoice templates plugin directory unless you want to modify it for some reason.
I’ve updated internal comments in the not null and not empty functions to explain the difference. Basically and item can be not null but empty (blank, 0, etc. are empty values). It appears there is no use of the not empty function but I’ll keep it around for now anyway.