Don't write out Subrs operator if array is empty
Don't write out Subrs operator if array is empty

Given that our C++ compiler just produces GlobalSubrs, this saves
bytes on fonts with many FDArrays, eg:
https://github.com/adobe-fonts/fdarray-test

moyogo/compreffor,googlefonts/compreffor,behdad/compreffor,googlei18n/compreffor,moyogo/compreffor,behdad/compreffor,googlefonts/compreffor,behdad/compreffor,googlei18n/compreffor,googlei18n/compreffor,moyogo/compreffor