@Override
    default ToDoubleNullable<ENTITY>
    mapToDoubleIfPresent(ToDoubleFunction<V> mapper) {
        return new FieldToDoubleImpl<>(this, mapper);
    }
Returns an {@link ToDoubleNullable} expression that has the value
returned by the specified mapper function if the value for this field is
not {@code null}, and otherwise {@code null}.

@param mapper  the mapper operation
@return  expression for this value after mapper has been applied

@since 3.1.0
runtime-parent/runtime-field/src/main/java/com/speedment/runtime/field/ReferenceField.java