Error connecting when user has no password

I am in the process of moving our application from postgresql 7.2.1-5 to
7.3.6-7. I also changed the jdbc driver we use from pg72jdbc2.jar to
pg73jdbc2.jar. I can no longer get a connection for users that have
no password from java. I couldn't figure out why I couldn't get a
connection with our usual user that has no database password, and then
on a hunch tried using a different user that has a password, and
everything worked fine. The no password user can connect using psql
with no problems.

The postgres upgrade is just one part of our platform update from
red hat 7.3 to enterprise linux 3, and java 1.4.2 to 5.0. So it could
possibly be something with the new java version as well, but the
exception I get is in the postgres jdbc driver code:

A connection error has occurred: FATAL: Password authentication failed for
user "admin"

at
org.postgresql.jdbc1.AbstractJdbc1Connection.openConnection(AbstractJdbc1Con
nection.java:202)
at org.postgresql.Driver.connect(Driver.java:122)
at java.sql.DriverManager.getConnection(DriverManager.java:525)
at java.sql.DriverManager.getConnection(DriverManager.java:171)
at
com.avamar.mc.datatap.ConnectionPool.createConnections(ConnectionPool.java:1
84)

ConnectionPool.java line 184 is basically this:

Connection con = DriverManager.getConnection( connectURL, "admin", "" );

This all worked fine with 7.2.1.5, and before that with some version of
7.1. Thanks in advance for any assistance.

Ed