Improve handling of alerts when no active alerts are present

docs/internal/weather-data.md

@@ -53,8 +53,9 @@ contract used by this project.
 
 The adapter records source name, endpoint, query, fetch time, source timestamps
 when available, SHA-256 hash over compact raw `data` JSON, missing status, and
-source warnings. `app.FetchAndSaveBundle` can write bundle JSON atomically for
-inspection.
+source warnings. Successful `data: null` responses from `/alerts/active`
+represent a checked empty active-alert list, not a missing source.
+`app.FetchAndSaveBundle` can write bundle JSON atomically for inspection.
 
 ## Skip And Resume Behavior
 
@@ -69,6 +70,8 @@ data is required and cannot be skipped.
   endpoint context.
 - Missing hourly data or hourly forecasts with no periods fail bundle fetch.
 - Optional and stub sources follow missing-source policy.
+- Explicit `data: null` from `/alerts/active` produces an empty, non-missing
+  alert run.
 
 ## Tests