indexer.cli: Fix missing retries_left parameter

According to the task schema, that parameter should not be null and defaults to
0. But current scheduler backend implementation expects a value nonetheless and
if it's null, that breaks the call.

This should fix [1]

[1]
https://jenkins.softwareheritage.org/job/DENV/job/tests/lastFailedBuild/artifact/swh-indexer.log