[ML] Data frame analytics feature importance decision path popover broken

[pheyos]

Found in version

• 7.12 / master

Browser

• Chrome

Steps to reproduce

• Open the results view for a classification job with enabled feature importance
• Open the decision path popover

Expected result

• The popover is displayed correctly

Actual result

• The popover displays []

Additional information

• The structure of the feature importance cell content changed (now contains item like "class_name":["no"] instead of "class_name":"no")
• This change has been introduced in Returning nested data in fields API elasticsearch#67432 (master) and Return structured nested data in ‘fields’ API elasticsearch#68580 (7.x/7.12)
• This change made the ES snapshot verification fail, so tests had to be disabled temporarily, see Failing ES promotion: machine learning data frame analytics total feature importance panel and decision path popover binary classification job should display the feature importance decision path in the data grid #90526

[elasticmachine]

Pinging @elastic/ml-ui (:ml)