Inconsistency behavior of CONCAT functions in difference databases

[isuru89]

CONCAT has different outcomes when NULL values are present in the concatenating value set.
For eg: MySQL returns NULL when there is at least one null argument, while other databases simply ignore null values.
This needs to be consistent over every database when using NyQL CONCAT function.

To keep the backward compatibility, we will introduce a different function called CONCAT_NE to return non-empty concatenated text always.