Conditional timer event

Added a note, explaining that when the timer expression evaluates to null/empty, then the timer scheduling is skipped and execution continues with the next activity.

Author: hugomendix

content/en/docs/refguide/modeling/application-logic/workflows/workflow-elements/timer.md

@@ -56,6 +56,10 @@ The **Timer** properties are described in the table below:
 | Duration | You can set a certain duration for the timer. With the **Continue after** setting, you can indicate the number of seconds, minutes, hours, days, weeks or months the timer's duration is. Possible values for the setting are:<br /><ul><li>Seconds</li><li>Minutes</li><li>Hours</li><li>Days</li><li>Weeks</li><li>Months</li> </ul> |
 | Expression | You can set a certain date and time for the timer by writing an expression via the **Continue at** setting.<br><br>For example, you can write `addDays([%CurrentDateTime%], 1)` to set tomorrow as the due date and time. To set a static date and time, you can use the expression `parseDateTimeUTC('2023-12-10T17:12:00.000', 'yyyy-MM-dd''T''HH:mm:ss.SSS')`.<br><br>You can also create a more complex timer. For example, you can set a timer based on a Boolean value (in this example, `isVIPUser`) from the provided workflow context entity: `if $WorkflowContext/isVIPUser then addDays([%CurrentDateTime%], 2) else addWeeks([%CurrentDateTime%], 2])`.<br><br>For more information on available expressions in Mendix, see [Expressions](/refguide/expressions/). |
 
+{{% alert color="info" %}}
+When the expression evaluates to `empty` or `null`, then no timer will be scheduled. In case of a standalone activity, execution continues at the next activity. In case of a timer boundary event, the boundary event is simply skipped.
+{{% /alert %}}|
+
 ### Recurrence Section {#recurrence}
 
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