fix: use config models when empty models list provided

When the trigger simulation API receives an empty models list ([]),
it now correctly falls back to enabled models from config instead
of running with no models.

Changes:
- Update condition to check for both None and empty list
- Add test case for empty models list behavior
- Update API documentation to clarify this behavior

All 28 integration tests pass.

tests/integration/test_api_endpoints.py

@@ -119,6 +119,18 @@ class TestSimulateTriggerEndpoint:
         data = response.json()
         assert data["total_model_days"] >= 1
 
+    def test_trigger_empty_models_uses_config(self, api_client):
+        """Should use enabled models from config when models is empty list."""
+        response = api_client.post("/simulate/trigger", json={
+            "start_date": "2025-01-16",
+            "end_date": "2025-01-16",
+            "models": []  # Empty list - should use enabled models from config
+        })
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["total_model_days"] >= 1
+
     def test_trigger_enforces_single_job_limit(self, api_client):
         """Should reject trigger when job already running."""
         # Create first job