Ok, I tried it with your other suggestion and it is working great now. Thank you for your help!
// Check if LOB field is not empty and if it includes a comma (,)
if (issue.fields.customfield_15600 && issue.fields.customfield_15600.length > 1)
{return “Multiple”;
} else {
// Return the actual field value if no comma
return issue.fields.customfield_15600;
}
// Return undefined if field is empty
return undefined;