[bug] Saved Objects export fails for documents without an updated_at field

[TinaHeiligers]

Exporting all saved objects fails if some objects don't have an updated_at field.

The saved object export API uses search_after where sort is based on updated_at field. If the export includes very old documents that were never updated, the bulk export can fail because updated_at is missing. The export API needs to be resilient against such an edge case.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[TinaHeiligers]

Exporting all objects from SOM sets the default sort field as updated_at with direction desc. There doesn't seem to be an option to change that so we need a reasonable alternative for very old saved objects that don't have an updated_at field.
We need to add a fallback or choose a different sort field default or fallback to the exportByTypes implementation that uses byIdAscComparator.