agent: Don't error when the agent navigation history hasn't been persisted (#35863)

This causes us to log an unrecognizable error on every startup otherwise

Release Notes:

- N/A

crates/agent/src/history_store.rs

@@ -212,7 +212,16 @@ impl HistoryStore {
     fn load_recently_opened_entries(cx: &AsyncApp) -> Task<Result<Vec<HistoryEntryId>>> {
         cx.background_spawn(async move {
             let path = paths::data_dir().join(NAVIGATION_HISTORY_PATH);
-            let contents = smol::fs::read_to_string(path).await?;
+            let contents = match smol::fs::read_to_string(path).await {
+                Ok(it) => it,
+                Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+                    return Ok(Vec::new());
+                }
+                Err(e) => {
+                    return Err(e)
+                        .context("deserializing persisted agent panel navigation history");
+                }
+            };
             let entries = serde_json::from_str::<Vec<SerializedRecentOpen>>(&contents)
                 .context("deserializing persisted agent panel navigation history")?
                 .into_iter()