@NotNull
    static MapAvailability getMapAvailability(final String mapName, final Set<String> mapNames) {
        final MapAvailability mapAvailability;
        if (mapNames == null) {
            // Not done a lookup yet so can't be sure what maps are in the stream, therefore
            // we must assume that a lookup is required.
            mapAvailability = MapAvailability.UNKNOWN;
        } else {
            // The DB has been queried to find out what maps exist, so we can be sure whether a lookup
            // is needed or not.  If the map is known to not exist then no point in doing a lookup.
            mapAvailability = mapNames.contains(mapName)
                    ? MapAvailability.PRESENT
                    : MapAvailability.NOT_PRESENT;
        }
        return mapAvailability;
    }
Check if the map exists in this {@link RefStreamDefinition} to determine if we need to bother
doing a lookup. If we haven't loaded refStreamDefinition yet then return UNKNOWN.
This saves having to do multiple pointless gets on the store to find there is no map
stroom-pipeline/src/main/java/stroom/pipeline/refdata/RefDataStoreHolder.java