getDimensionIdFromNaturalKey should return '_NF_' when no Dimension record found

Currently the 'getDimensionIdFromNaturalKey' function in DimensionServices.java returns the DimensionId when found with following code excerpt

if (UtilValidate.isNotEmpty(lastDimensionValue)) {
            resultMap.put("dimensionId", lastDimensionValue.getString("dimensionId"));
        }
return resultMap;

And when not found it returns an empty result, leaving the using function upstream to set the '_NF' value.

The 'getDimensionIdFromNaturalKey' function should return the 'NF' value when no matching dimensionId is found.

This would simplify the upstream code.