I believe the CHARINDEX returns 0 if the delimiter is not found in the string. Tweaked the customer number formula a little bit to handle these scenarios

CASE WHEN coalesce(byte#_of_last_dash, 0) = 0 THEN '' <<-- handles if original string is NULL or not found

WHEN byte#_of_last_dash = 1 THEN '' <<-- Handles when the delimiter is the last character in the orginal string

ELSE LTRIM(RTRIM(RIGHT(original_string, byte#_of_last_dash - 1)

END AS customer_number