[ML] Edit transform retention policy fails with default field value

[pheyos]

Found in version

• 7.16.2
• Could also reproduce in main / 8.1

Browser

• Chrome

Steps to reproduce

• Create a transform without retention policy
• Edit the transform and add a retention policy while not changing the pre-filled / default value of the Field dropdown

Expected result

• The transform update can be saved correctly

Actual result

• An error occurs during transform update as the field is not sent in the request:

  Bad Request: [request body.retention_policy.time.field]: expected value of type [string] but got [undefined]
  

  

Additional information

• Workaround: The user has to select a different value from the dropdoiwn first and the re-select the initial value to make it work.
• If there's no other time field available, it seems there's no workaround for this issue on the UI side and the user would have to use the ES API.
• The wizard component for retention policy is not affected and works fine with the initial value

[elasticmachine]

Pinging @elastic/ml-ui (:ml)