[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.
5 jobs for 26_fix_end_index_handling_in_write in 14 minutes and 26 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build_linux_36 #174778
docker

00:07:49

73.0%
passed build_linux_37 #174782
docker

00:06:28

73.0%
passed build_macosx_36 #174776
macosx

00:03:59

73.0%
passed build_macosx_37 #174777
macosx

00:03:59

73.0%
failed build_linux_37 #174779
docker

00:03:17