Use serde(skip) instead of skip_serializing, add placeholder values (#3362)

Use serde(skip) instead of skip_serializing, add placeholder values (#3362)

* Use serde(skip) instead of skip_serializing

The latter breaks lemmy_crawler as the field is not included in
the Lemmy API, but is required when attempting to parse API responses.
Should only use serde(skip) to avoid this problem

* use option

* add placeholders

* no unwrap