Description

When PPSTRESU is NA for all rows in a parameter group (e.g., partial AUC intervals like RCAMINT where no result was computed), multiple things break:

1. Descriptive statistics crash — calculate_summary_stats() crashes at line 67:

ModeUnit = names(sort(table(PPSTRESU), decreasing = TRUE, useNA = "ifany")[1])[1]

table() drops NAs by default, returning an empty table. names(sort(empty_table, ...)[1])[1] returns NULL, and dplyr::mutate cannot combine NULL with character results across groups:

Error in mutate:
! `ModeUnit` must return compatible vectors across groups.
✖ Can't combine NULL and non NULL results.

2. Column names get NA suffix — In pivot_wider_pknca_results.R and label_operators.R, the pattern ifelse(PPSTRESU != "", ...) returns NA when PPSTRESU is NA (since NA != "" evaluates to NA). This produces column names like RCAMINT_0-20NA instead of RCAMINT_0-20.

Both issues stem from export_cdisc.R line 394 setting PPSTRESU = ifelse(is.na(PPSTRES), NA_character_, PPSTRESU), which is correct CDISC behavior but wasn't handled downstream.

To reproduce

1. Upload a dataset with both SERUM and URINE specimens.
2. In NCA settings, add partial AUC intervals (e.g., AUCINT and RCAMINT).
3. Run NCA computation.
4. In NCA Results > Select Parameters, observe RCAMINT_0-20NA instead of RCAMINT_0-20.
5. Navigate to Descriptive Statistics — app crashes.

The issue is on main.

First reported in #1169 (review).

Expected behaviour

• Descriptive statistics should handle groups where all unit values are NA without crashing.
• Manual interval parameters should appear as RCAMINT_0-20 (without NA suffix) in NCA results.