[cp2fm] empty configurations have to be included in the resulting Feature Model

Currently, empty configurations are filtered out by the converter, see https://github.com/apache/sling-org-apache-sling-feature-cpconverter/blob/master/src/main/java/org/apache/sling/feature/cpconverter/handlers/AbstractConfigurationEntryHandler.java#L61

They have to be included anyway otherwise they conflict with service which have ConfigurationPolicy.REQUIRE