Sorting nulls and empty strings together

User interface need to show nulls as empty strings.
PostgreSQL sorts nulls after all data.

create temp table test ( testcol char(10) );
insert into test values ( null);
insert into test values ( 'test');
insert into test values ( '');
select * from test order by testcol;

This confuses users who expect that all empty columns are together in sorted
data.

Select statements are generated dynamically by driver and it is not easy
to change them to generate order by coalesce( testcol,'').
If there is no other way I can change driver to generate coalesce(
testcol,'') as order by expressions.
However I'm afraid that those order by expression cannot use regular index
like

create index test_inx on test(testcol)

in it thus too slow for large data.

How to force PostgreSQL to sort data so that nulls and empty strings appear
together ?

Andrus.