bug with iterator return from tcpsock:receiveuntil

**Describe the bug**
the iterator return from tcpsock:receiveuntil in your ngx.lua
`---@alias ngx.socket.tcpsock.iterator fun(size:number):string,string,any`
should be
`---@alias ngx.socket.tcpsock.iterator fun(size:number|nil):string,string,any`
see [receiveuntil](https://github.com/openresty/lua-nginx-module/blob/master/README.markdown#tcpsockreceiveuntil)
