17281 FIX Skip creation of invalid metric

When a user without an ID was present in the monitoring the `check_notify_count` active
check would be able to create an invalid metric called `_num`.

Now this is no longer possible and the metric is just being skipped.

SUP-19694

Change-Id: If01fba2a8440ccbe2114997d50a3f67deb815df7

.werks/17281.md

@@ -0,0 +1,17 @@
+[//]: # (werk v2)
+# Skip creation of invalid metric
+
+key        | value
+---------- | ---
+date       | 2025-02-13T13:26:51+00:00
+version    | 2.5.0b1
+class      | fix
+edition    | cre
+component  | checks
+level      | 1
+compatible | yes
+
+When a user without an ID was present in the monitoring the `check_notify_count` active
+check would be able to create an invalid metric called `_num`.
+
+Now this is no longer possible and the metric is just being skipped.

active_checks/check_notify_count

@@ -87,6 +87,9 @@ details = []
 num_users = 0
 raw_notifications = 0
 for contact_name, num in c.query_table(query):
+    # This results in an invalid metric and should therefore be skipped.
+    if contact_name == "":
+        continue
     if contact_name == "check-mk-notify":
         raw_notifications += num
     else: