OF ''

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: I5a8356656ea0b7ab483fef0e3a05dc84e63c9f1a