Your link to your persistence XML does not work, perhaps include it.
The null pointer is occuring in your JDBC driver (Derby) not EclipseLink.
The error is occuring on connect, so my guess would be you did not set a
user name or password. You must set a user name and password. JPA does
not have standard properties for these in the persistence.xml, for
EclipseLink they are "eclipselink.jdbc.user", "eclipselink.jdbc.password".