looking for empty fields

We recently upgraded from version 7.2.3 to 7.3.3 and seem to have some
code that has broken. We were doing checks in various locations looking
for nulls and/or empty fields. To search for empty fields we said
something like

select count(am_pk) from arraymeasurement where al_fk is null or al_fk='';

Is this not allowed anymore? All my check with the double ticks are
failing. I assume there is a difference in postgres between an empty and
null field. How can I check for both in 7.3.3?
Thanks
Jodi
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_______________________________
Jodi L Kanter
BioInformatics Database Administrator
University of Virginia
(434) 924-2846
jkanter(at)virginia(dot)edu <mailto:jkanter(at)virginia(dot)edu>