The concatenation operator || in Oracle skips all NULL arguments and returns the result of concatenation of non-NULL arguments.
NULL is returned only if all arguments were NULL.
Under terms of this task we'll make the || operator work in Oracle style when sql_mode is ORACLE.
Note, the function CONCAT won't be changed by this task.
There is a contributed patch from Jérôme Brauge implementing this feature:
It adds a new flag MODE_CONCAT_NULL_YIELDS_NULL_OFF.
We'll probably won't add the flag and just perform the Oracle style concatenation when sql_mode & MODE_ORACLE is set.