fix issue when next_task_nodes is None

[sanderegg]

What do these changes do?

• fix issue when next_task_nodes is empty

Related issue number

How to test

Checklist

• Did you change any service's API? Then make sure to bundle document and upgrade version (make openapi-specs, git commit ... and then make version-*)
• Unit tests for the changes exist
• Runs in the swarm
• Documentation reflects the changes
• New module? Add your github username to .github/CODEOWNERS

[codecov]

Codecov Report

Merging #1411 into master will decrease coverage by 21.3%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #1411      +/-   ##
=========================================
- Coverage    71.2%   49.9%   -21.3%     
=========================================
  Files         237     123     -114     
  Lines        9371    4398    -4973     
  Branches     1027     475     -552     
=========================================
- Hits         6673    2195    -4478     
+ Misses       2398    2107     -291     
+ Partials      300      96     -204

Flag	Coverage Δ
#integrationtests	?
#unittests	49.9% <0%> (-12.31%)	⬇️

Impacted Files	Coverage Δ
...ices/sidecar/src/simcore_service_sidecar/celery.py	0% <0%> (ø)	⬆️
...ices/sidecar/src/simcore_service_sidecar/config.py	0% <0%> (-100%)	⬇️
...es/sidecar/src/simcore_service_sidecar/rabbitmq.py	0% <0%> (-95.09%)	⬇️
services/sidecar/src/simcore_service_sidecar/db.py	0% <0%> (-92%)	⬇️
...vices/sidecar/src/simcore_service_sidecar/utils.py	0% <0%> (-87.18%)	⬇️
.../sidecar/src/simcore_service_sidecar/log_parser.py	0% <0%> (-84.22%)	⬇️
...rvices/sidecar/src/simcore_service_sidecar/core.py	0% <0%> (-81.23%)	⬇️
...ervices/sidecar/src/simcore_service_sidecar/cli.py	0% <0%> (-74.08%)	⬇️
...re-sdk/src/simcore_sdk/node_ports/serialization.py	24.61% <0%> (-56.93%)	⬇️
.../sidecar/src/simcore_service_sidecar/exceptions.py	0% <0%> (-55.56%)	⬇️
... and 124 more

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[pcrespov]

MINOR: could you perhaps specify a bit more about the nature of the exception :-) I am aware that the exception traceback is printed but still...