Handle null and empty valued response headers

[JunTaoLuo]

Similar to aspnet/KestrelHttpServer#429, setting null values in a multi-valued known header will crash the server.

The expected behaviour is null values for headers should be ignored both for known and unknown headers. Header values of empty strings will be ignored for known headers due to API limitations but will be sent in the response for unknown headers.

[JunTaoLuo]

To reduce unnecessary filtering, it was decided to convert all null headers to empty headers and let Http.Sys decide whether it's included in the response. Currently, Http.Sys will ignore null/empty known headers but will keep null/empty unknown headers.