Add impl Default for SdkBody

[Velfi]

Implement the Default trait for the SdkBody struct, preferably by having it call SdkBody::empty

[rcoh]

so because of how SDKBody, works, default would be counter intuitive in one place, where we swap the body for taken() to record that the body ownership was moved elsewhere.

If SdkBody implemented Default, then you could use mem::take which would produce unexpected results.