std.os.accept already wants to allow null, which matches `man 3p accept`:

>  address     Either a null pointer, or a pointer to a sockaddr structure
>              where the address of the connecting  socket  shall  be  re‐
>              turned.
>
>  address_len Either  a  null pointer, if address is a null pointer, or a
>              pointer to a socklen_t object which on input specifies  the
>              length  of  the  supplied sockaddr structure, and on output
>              specifies the length of the stored address.

Fixes ziglang#6832.