[nginx-quic] fix qpack header null value issue

[Sergey Kandaurov]

> On 28 May 2021, at 07:02, sun edward <sunzhiyong3210 at gmail.com> wrote:
> 
> description:
>      when header with a null value,need to reset st->value,otherwise it is taking previous header field's value
> 

Thanks, a slightly different version committed:
https://hg.nginx.org/nginx-quic/rev/3509b9dcfb47

[..]

-- 
Sergey Kandaurov