Geckoprofiler visualProgress timestamp is null

In the Geckoprofiler.json the visual metrics progress field is broken as reported in https://github.com/firefox-devtools/profiler/issues/3966#issuecomment-1084300943

"visualMetrics":{"FirstVisualChange":766,"LastVisualChange":2200,"SpeedIndex":832,"VisualProgress":[{"timestamp":null,"percent":0},{"timestamp":null,"percent":1},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100},{"timestamp":null,"percent":100}],"videoRecordingStart":3900,"VisualReadiness":1434,"VisualComplete85":833,"VisualComplete95":833,"VisualComplete99":833}}

There are some massaging of the data that seems broken in:
https://github.com/sitespeedio/browsertime/blob/main/lib/firefox/webdriver/firefox.js#L193-L230

Inspect lib/firefox/webdriver/firefox.js around lines 193-230 and compare its data massaging with the Geckoprofiler.json example and the referenced profiler discussion. Reproduce or trace the VisualProgress handling, then verify that generated VisualProgress entries contain valid timestamps rather than null values.