private static List<SingleRefDataValueProxy> getContainedProxies(final RefDataValueProxy refDataValueProxy) {
        Objects.requireNonNull(refDataValueProxy);
        if (refDataValueProxy instanceof SingleRefDataValueProxy) {
            return List.of((SingleRefDataValueProxy) refDataValueProxy);
        } else if (refDataValueProxy instanceof MultiRefDataValueProxy) {
            return ((MultiRefDataValueProxy) refDataValueProxy).refDataValueProxies;
        } else {
            throw new RuntimeException(LogUtil.message("Unexpected type " + refDataValueProxy.getClass().getName()));
        }
    }
Merge the two proxies into one combined one that will use each contained proxy to try to get a value.
Argument order is CRITICAL to ensure lookups are done in the correct order.

@param existingRefDataValueProxy The existing
@param newRefDataValueProxy
@return
stroom-pipeline/src/main/java/stroom/pipeline/refdata/store/MultiRefDataValueProxy.java