Configuration file when empty returns AttributeError: 'NoneType' object has no attribute 'get'

During tests on swh-indexer When debugging with @olasd the new tox command First I had the following error: swh/meta$326 resolved with https://wiki.postgresql.org/wiki/Apt#Quickstart

then I had ImportError: No module named 'swh.deposit' because in the configuration file there were tasks that shouldn't be dealt during tests on swh-indexer

Lastly when keeping a worker.yml empty file, I had : AttributeError: 'NoneType' object has no attribute 'get'

This was only solved when the file wasn't emty: task_modules: [ ]

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Migrated from T1299 (view on Phabricator)