[execution][messagehandlers] Fix write end_data_index handling in LoopMessageHandler

The None value is not properly handled and ends up triggering a warning while it is a valid value for end_data_index.

This patch fixes this.

added bug confirmed labels

changed milestone to %Soft loops

Note that this is not critical, it will simply show the warning but it is misleading in this case.

enabled an automatic merge when the pipeline for 9c5b097c succeeds

merged

mentioned in commit 18f947a2