This is actually tricker than I thought at first.   If there is only one value passed in, even if its from a multivalue attribute, it passes it in as a simple string.  If there is more than one value passed in, then it gives you a string, however its a json encoded array.    That's not fun.

This example below will detect if it thinks its array (if the first char is a bracket '[') and then join all the values together.  If the first char isn't a bracket it just sends back the inputValue.

In your case you could modify this to just set outputValue to the the jObject[0] value I think after the JSON.parse is finished.

--Jim

if(inputValue[0] == '[' && inputValue[1]== '"') {
    var result = '';
    var jObject = JSON.parse(inputValue);
    for (var content in jObject) {
        result += jObject[content] + ' ';
    }
    outputValue = result;
} else {
    outputValue = inputValue;
}

Also, I've used this snippet in the back to avoid the warning in the collection, but to give me feedback when a multivalue is still in there (usually after the app owner SWEARS that there aren't any, and they appear to want to clean them all up):

if(inputValue[0] == '[') {
    outputValue = 'Multivalue detected';
} else {
    outputValue = inputValue;
}

Thank you! It helped me a lot.