Use skip_serializing_none for nodeinfo (fixes #4702)

Cargo.lock

@@ -2940,6 +2940,7 @@ dependencies = [
  "reqwest-middleware",
  "rss",
  "serde",
+ "serde_with",
  "tokio",
  "tracing",
  "url",

crates/routes/Cargo.toml

@@ -34,4 +34,5 @@ once_cell = { workspace = true }
 tracing = { workspace = true }
 tokio = { workspace = true }
 urlencoding = { workspace = true }
+serde_with.workspace = true
 rss = "2.0.7"

crates/routes/src/nodeinfo.rs

@@ -9,6 +9,7 @@ use lemmy_utils::{
   VERSION,
 };
 use serde::{Deserialize, Serialize};
+use serde_with::skip_serializing_none;
 use url::Url;
 
 pub fn config(cfg: &mut web::ServiceConfig) {
@@ -44,7 +45,7 @@ async fn node_info(context: web::Data<LemmyContext>) -> Result<HttpResponse, Err
     .ok_or(ErrorBadRequest(LemmyError::from(anyhow!("not_found"))))?;
 
   let protocols = if site_view.local_site.federation_enabled {
-    Some(vec!["activitypub".to_string()])
+    None
   } else {
     None
   };
@@ -84,6 +85,7 @@ struct NodeInfoWellKnownLinks {
   pub href: Url,
 }
 
+#[skip_serializing_none]
 #[derive(Serialize, Deserialize, Debug, Default)]
 #[serde(rename_all = "camelCase", default)]
 pub struct NodeInfo {
@@ -94,6 +96,7 @@ pub struct NodeInfo {
   pub open_registrations: Option<bool>,
 }
 
+#[skip_serializing_none]
 #[derive(Serialize, Deserialize, Debug, Default)]
 #[serde(default)]
 pub struct NodeInfoSoftware {
@@ -101,6 +104,7 @@ pub struct NodeInfoSoftware {
   pub version: Option<String>,
 }
 
+#[skip_serializing_none]
 #[derive(Serialize, Deserialize, Debug, Default)]
 #[serde(rename_all = "camelCase", default)]
 pub struct NodeInfoUsage {
@@ -109,6 +113,7 @@ pub struct NodeInfoUsage {
   pub local_comments: Option<i64>,
 }
 
+#[skip_serializing_none]
 #[derive(Serialize, Deserialize, Debug, Default)]
 #[serde(rename_all = "camelCase", default)]
 pub struct NodeInfoUsers {