default <E extends Enum<E>> ToEnumNullable<ENTITY, E>
    mapToEnumIfPresent(Function<V, E> mapper, Class<E> enumClass) {
        return new FieldToEnumImpl<>(this, mapper, enumClass);
    }
Returns an {@link ToEnumNullable} 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 <E>        the enum type
@param mapper     the mapper operation
@param enumClass  class of the enum to map to

@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