Internal helper to convert a list of NordBord test records into a tidy data.frame.
For character columns, NULL values are replaced with an empty string ("")
to ensure consistent downstream handling.
.build_nordbord_df(records)A data.frame with one row per NordBord test record.
Internal function (not designed to be used directly by end users)
A list of NordBord test records from the API.
Special handling is applied for the identifier column: some NordBord API endpoints
return this as profileId, while others return it as athleteId.
This function will populate the athleteId column by taking the first non-empty value
from athleteId or profileId, in that order.