Issue:

The string functions 'concat' and 'concat_ws' return a default
String(with null data pointer) when they have to return empty
string. This propagates as a NULL value in the result.

Fix:

Return a empty String("") for empty results from the string functions
'concat' and 'concat_ws'.

Change-Id: I81a15f31b84cbad68796e96db80d6aae2d3ae0e5