[ML] Anomaly Detection: Distribution Charts fail to load for complex detectors

[walterra]

To reproduce, create a job with gallery sample data like:

rare by status over clientip
freq_rare by uri over clientip
high_count by status over clientip
high_count by uri over clientip
sum(bytes) by method over clientip

1 hour bucket span

influencers: clientip, file

For some anomalies in Anomaly Explorer, the anomaly charts fail to load details like background distribution or pick the wrong chart type.

For example, here the correct charts show up:

Here some charts stay empty. Also not that the rare detectors shows a line chart whereas it should show the special chart for rare detectors:

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Fixed the issue with the blank population charts in #84254.

Note that the rare by status over clientip chart in the screenshot above is behaving as intended, where we currently use the metric line chart for rare detectors with an over field, plotting count. There is an open meta issue #21163 to investigate enhancements for rare detectors in population analyses.