pom.xml: update killbill-oss-parent to 0.4.2

The main changes are around namespacing of notification queue properties
and workarounds to support the Metrics upgrade.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
currency conversion: don't populate converted values if the source table is empty

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>