[backend][experimentmodel] Fix DatasetBlock output handling

Fixes #268
4 jobs for 268_fix_datasetblock_missing_output_handling in 10 minutes and 22 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #201042
docker
build_linux_36

00:10:05

90.0%
passed #201043
docker
build_linux_37

00:09:45

90.0%
passed #201040
macosx
build_macosx_36

00:10:18

90.0%
passed #201041
macosx
build_macosx_37

00:10:02

90.0%