ffi: quiche_conn_close and quiche_conn_stream_send check for null pointers (fix #1914)

[ichordev]

Functions quiche_conn_close and quiche_conn_stream_send now check to make sure the passed pointers are not null before creating a slice with them. If they are null, &[] is used instead. Length must be 0 when the buffer is null, of course.

• Fixes issue ffi: client.c panics on quiche_conn_close because reason is NULL #1914.
• Allows passing null to quiche_conn_stream_send for convenience to trigger FIN STREAM.

[ghedo]

Merged, thanks @ichordev!