[exim] sqlite lookups and .nullvalue

Hi,

I've a sqlite lookup which executes a query like this:

select group_concat(recipients) from aliases where stem = "blah";

The problem is, when there are no matches, the result in exim is the string "NULL".

I see there is a config option in the sqlite command line client:

    .nullvalue STRING      Print STRING in place of NULL values

Is it possible to modify this setting in exim so that my lookups result in ""?
I can't see any indication that there is in the manual or googling generally.

Thanks?

Nick