[ML] Handle Empty Partition Field Values in Single Metric Viewer

[blaklaybul]

Jobs with partition fields created via the API skip validation checks and therefore can analyze data where the partition field does not exist. This causes the anomaly explorer to show a single swimlane with an empty value for the partition field. If you jump into the single metric viewer, you are unable to select this empty value from the dropdown.

anomaly explorer:

single metric viewer:

Ideally, the ML UI should indicate that this is field either does not exist in the datafeed, or it is empty (i.e. "") for all docs returned by the datafeed. The user should also be able to select this from the dropdown in the single metric viewer.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)