default ToCharNullable<ENTITY>
    mapToCharIfPresent(ToCharFunction<V> mapper) {
        return new FieldToCharImpl<>(this, mapper);
    }
Returns an {@link ToCharNullable} 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