AIF Check on Check Table for non mandatory fields

Hi AIF Community,

I need to implement following Check:

I have a simple 1:1 mapping and need to check the raw data value (STRING type) against a check table. But the target field is an optional field. That means in addition to the check table content an empty field is also allowed. If the value is not in the check table the check will fail. The simple Check set up is as follows:

I didn’t found a possibility to set up the Check only by customizing so that empty fields are also allowed.

Does someone know if there is any other option beside developing a function module for each field?

Kind regards

Christoph