[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 Job ID Name Coverage
  Build
passed #174778
docker
build_linux_36

00:07:49

73.0%
passed #174782
docker
build_linux_37

00:06:28

73.0%
passed #174776
macosx
build_macosx_36

00:03:59

73.0%
passed #174777
macosx
build_macosx_37

00:03:59

73.0%
failed #174779
docker
build_linux_37

00:03:17