In this case, it means we are providing an empty string as an argument to the CHAR_LENGTH() function. It will return 0 on providing empty string because there are no characters to be counted by CHAR_LENGTH() function.
Example
mysql> Select CHAR_LENGTH(''); +-----------------+ | CHAR_LENGTH('') | +-----------------+ | 0 | +-----------------+ 1 row in set (0.00 sec)