[tex-hyphen] zerohyph left/right should not be empty

[Karl Berry]

Arthur or anyone - please change your texhyphen tlpsrc generation so
that the values for the zerohyph languages (arabic, farsi) are zero, not
the empty string. Just for cleanliness.

Report: https://tug.org/pipermail/tex-live/2021-May/046962.html

--thanks, karl.