Default serde on a couple Ready structfields
Default serde on a couple Ready structfields

Have serde default on the `presences` and `private_channels`
structfields of `Ready`. Some JSON serializers might leave these out
when serializing if they're empty, so resolve this by simply defaulting
to empty maps.

Lakelezz/serenity,zeyla/serenity.rs,Flat/serenity,zeyla/serenity,acdenisSK/serenity