[ML] Errors creating analytics job if array of indices or query is configured in editor

[peteharverson]

Kibana version: 7.6.0

When creating a data frame analytics job, and switching to the advanced editor, if an array of indexes is entered into the config (rather than a single index as a string), creation of the job fails with the following error displayed in the flyout:

Supplying a query also results in an error:

This may be a result of the route schema validation added in #53521.

Extra functional tests should also be added to /x-pack/test/functional/apps/machine_learning/data_frame_analytics to cover these use cases.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)