>>that if write(2) sees a zero length argument, it should consider that a
>>nop and return.  But then it seems that that would break some weird
>>semantic somewhere (``Any write will update a timestamp; except for
>>zero length writes, which are ignored.'' ``Dude, that's broken.'')

it isn't only the possible effect on time stamps; i think it
should do a write 0 because it's what you said to do.
to be fair, it's disguised by print, but even so,
if i wanted nop() i'd use
	;
or
	void nop(void){}
but i wouldn't call write() or even print.  i'd call write() when i wanted to write,
and i prefer systems that do what i tell it, so that if i say `write 0 bytes'
i expect it to do that, rather than adding yet
more special cases to remember.   the effect of writing 0 bytes
is determined by the device, but that's true of writing 1 byte or many,
so there's no difference there.

on the other hand, i'd probably write
	if(*str)
		print("%s", str);
if that's what i wanted.