Allow empty observations in alerting (#10939)

Co-authored-by: bogdan kyryliuk <bogdankyryliuk@dropbox.com>

superset/tasks/alerts/observer.py

@@ -48,7 +48,7 @@ def observe(alert_id: int, session: Session) -> Optional[str]:
 
     error_msg = validate_observer_result(df, alert.id, alert.label)
 
-    if not error_msg and df.to_records()[0][1] is not None:
+    if not error_msg and not df.empty and df.to_records()[0][1] is not None:
         value = float(df.to_records()[0][1])
 
     observation = SQLObservation(
@@ -74,9 +74,9 @@ def validate_observer_result(
     Returns an error message if the result is invalid.
     """
     try:
-        assert (
-            not sql_result.empty
-        ), f"Observer for alert <{alert_id}:{alert_label}> returned no rows"
+        if sql_result.empty:
+            # empty results are used for the not null validator
+            return None
 
         rows = sql_result.to_records()
 

tests/alerts_tests.py

@@ -128,11 +128,11 @@ def test_alert_observer(setup_database):
     assert alert3.sql_observer[0].observations[-1].value is None
     assert alert3.sql_observer[0].observations[-1].error_msg is None
 
-    # Test SQLObserver with empty SQL return
+    # Test SQLObserver with empty SQL return, expected
     alert4 = create_alert(dbsession, "SELECT first FROM test_table WHERE first = -1")
     observe(alert4.id, dbsession)
     assert alert4.sql_observer[0].observations[-1].value is None
-    assert alert4.sql_observer[0].observations[-1].error_msg is not None
+    assert alert4.sql_observer[0].observations[-1].error_msg is None
 
     # Test SQLObserver with str result
     alert5 = create_alert(dbsession, "SELECT 'test_string' as string_value")