[experiments][js][panels] Handle empty choice case for string field

String fields should provide all the possible values that
a user can select. But if for some reason the field is missing,
the interface loading will break. This patch fixes that by
setting the default value as only choice.
1 job for experiment_panels_empty_choice_handling in 26 minutes and 45 seconds (queued for 14 minutes and 34 seconds)