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