`cb.correct.aipw_cComBat(...)` did not throw the expected warning.

"Matching cComBat makes outcomes for each group more similar"
"Matching cComBat raises warning when few samples are retained"
"Matching cComBat raises error when no samples are retained"
"apply Matching cComBat raises error when oos data has inconsistent batches"
"AIPW cComBat makes outcomes for each group more similar"
"AIPW cComBat raises warning when few samples are retained"
"AIPW cComBat raises error when no samples are retained"
"apply AIPW cComBat works reasonably well"
"apply AIPW cComBat raises error when new data has inconsistent batches"

AIPW cComBat raises warning when few samples are retained