## Summary
Fix a bug that was causing the cluster to fall into an unrecoverable
state if an error occurs during the document migration phase of a `v2`
to `zdt` migration (first zdt migration to take over the `v2`
algorithm).
When an error occurs during the document migration phase, the index gets
in a state where `mappingVersions` is present but not `docVersions`, and
the algorithm wasn't able to understand what this state was, and
considered it to just be a plain `zdt` predecessors, which caused an
error when entering the document migration phase, as `docVersions`
couldn't be found.
This PR addresses this, by properly identifying this specific state, and
acting accordingly (by initiating a document migration without checking
for the versions in `docVersions`, as we do then coming from a plain v2
algo)
79b5754a12