quick_setup: don't allow selecting of host/service without adding events

In stage 1: Triggering events, it was possible to check the
Host/Service events checkbox but not select any events for
them.  This change modifies the already existing validator
to catch this.

Change-Id: I9e45967c492b5436e2870fd1ccddf47315d0b05b

cmk/gui/wato/pages/notifications/quick_setup.py

@@ -390,7 +390,7 @@ def custom_recap_formspec_triggering_events(
 def _validate_empty_selection(selections: Sequence[Sequence[str | None]]) -> None:
     # TODO validation seems not to be possible for a single empty element of
     # the Tuple
-    if ["", None] in selections:
+    if ["", None] in selections or not selections:
         raise ValidationError(
             Message("At least one selection is missing."),
         )