fix: uv.write(pipe, {}) trigger assert(nbufs > 0)

[phanen]

uv.write accept string table but don't accept empty table.

local uv = require('luv')
uv.write(uv.new_pipe(false), {})

(assert: https://github.com/libuv/libuv/blob/3a9a6e3e6bc78565ddf94cf462c9877c1004bb62/src/unix/stream.c#L1298)

Using host libthread_db library "/usr/lib/libthread_db.so.1".
Core was generated by `/usr/bin/luajit a.lua'.
Program terminated with signal SIGABRT, Aborted.
#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
44            return INTERNAL_SYSCALL_ERROR_P (ret) ? INTERNAL_SYSCALL_ERRNO (ret) : 0;
(gdb) bt
#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
#1  0x00007f48e41f3813 in __pthread_kill_internal (threadid=<optimized out>, signo=6) at pthread_kill.c:89
#2  0x00007f48e4199dc0 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007f48e418157a in __GI_abort () at abort.c:73
#4  0x00007f48e41814e3 in __assert_fail_base (fmt=<optimized out>, assertion=0x7f48e4020969 "nbufs > 0", file=<optimized out>, line=<optimized out>,
    function=<optimized out>) at assert.c:118
#5  0x00007f48e3fe6273 in uv_write2[cold] () from /home/phan/.luarocks/lib/lua/5.1/luv.so
#6  0x00007f48e3ffcbe8 in luv_write () from /home/phan/.luarocks/lib/lua/5.1/luv.so
#7  0x00007f48e4352f06 in lj_BC_FUNCC () at buildvm_x86.dasc:857
#8  0x00007f48e4369a90 in lua_pcall (L=L@entry=0x7f48e43ea380, nargs=nargs@entry=0, nresults=nresults@entry=-1, errfunc=errfunc@entry=2)
    at /usr/src/debug/luajit/LuaJIT-f9140a622a0c44a99efb391cc1c2358bc8098ab7/src/lj_api.c:1122
#9  0x000055890091e504 in docall (L=L@entry=0x7f48e43ea380, narg=narg@entry=0, clear=clear@entry=0)
    at /usr/src/debug/luajit/LuaJIT-f9140a622a0c44a99efb391cc1c2358bc8098ab7/src/luajit.c:137
#10 0x000055890091f656 in handle_script (L=0x7f48e43ea380, argx=<optimized out>)
    at /usr/src/debug/luajit/LuaJIT-f9140a622a0c44a99efb391cc1c2358bc8098ab7/src/luajit.c:307
#11 pmain (L=0x7f48e43ea380) at /usr/src/debug/luajit/LuaJIT-f9140a622a0c44a99efb391cc1c2358bc8098ab7/src/luajit.c:565
#12 0x00007f48e4352f06 in lj_BC_FUNCC () at buildvm_x86.dasc:857
#13 0x00007f48e4369af3 in lua_cpcall (L=L@entry=0x7f48e43ea380, func=func@entry=0x55890091ee00 <pmain>, ud=ud@entry=0x0)
    at /usr/src/debug/luajit/LuaJIT-f9140a622a0c44a99efb391cc1c2358bc8098ab7/src/lj_api.c:1179
#14 0x000055890091e079 in main (argc=2, argv=0x7ffeb2f70308) at /usr/src/debug/luajit/LuaJIT-f9140a622a0c44a99efb391cc1c2358bc8098ab7/src/luajit.c:596