javaunohelper: String comparison

Change-Id: Ic0291e655525639b2f3cd33760d8f5f343b208f1
Reviewed-on: https://gerrit.libreoffice.org/10638
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>