Subject / security domain overrides null values from CRI for a datasource with allow-multiple-users

An issue cloned from JBPAPP6-1323 for the upstream.

DataSource configured with security-domain using org.picketbox.datasource.security.CallerIdentityLoginModule is able to create correct connection
when one of ds.getConection(username, password) is set to null.
Null parameter is substituted using value of corresponding field from the security-domain.
The datasource is defined using <allow-multiple-users/>.

See test org.jboss.as.test.integration.jca.security.DsWithCallerIdentityLoginModuleTestCase at https://github.com/pskopek/jboss-as/tree/JBPAPP-9583.