fix: address pre-push review findings

dashboard/server.py

@@ -32,6 +32,7 @@ from scoring import engine
 from dashboard.persistence import (
     append_daily_jsonl,
     atomic_write_json,
+    has_observation,
     load_json,
     load_jsonl_tail,
     merge_observation,
@@ -293,7 +294,7 @@ def run_scrape(force_full=False):
                     existing_cache.get(key), onchain.get(key), source=source,
                     error="metric missing from scrape",
                 )
-            if successful_sources:
+            if successful_sources and any(has_observation(value) for value in onchain.values()):
                 metrics["_onchain_timestamp"] = datetime.now(timezone.utc).isoformat()
             elif "_onchain_timestamp" in existing_cache:
                 metrics["_onchain_timestamp"] = existing_cache["_onchain_timestamp"]