Upgrade to 3.0.0 fails under some circumstances

Context:
In some circumstances the upgrade to 3.0.0 from 2.7.0 fails with a NullPointerException, see e.g. SUPPORT-7639. It could get worked around by deleting the event index created by the 3.0.0 upgrade though with

curl -s -XDELETE localhost:9200/optimize-event_v2-000001

But we need to investigate the actual cause, the NPE seems to be related to the `writeIndex` flag of the event index being null.

AT:

• 3.0.0 docs mention this known issue when upgrading sequentially from <=2.6.0 all the way up to 3.0.0
  • this needs to get merged to master, latest and 3.0.0 branches
• the potential NPE on a write index flag missing is fixed in `UpdateIndexStep`, it just defaults to true in this case
• the `CreateIndexStep` uses the `ElasticSearchSchemaManager.createOptimizeIndex` method to create indexes