'Node Missing' Message when field is in REST Response, but is empty

We have test scenarios which require us to validate when a given element is not passed in the request, the value for that element is empty in the response echo. We can see the element is returned in the echo and that its value is empty, which is the desired outcome. Our JSON and XML Message Content Assertions are failing in these scenarios with a 'Node Missing' message. I have read about this issue on several other forum posts and it seems the suggested solution is to write script assertions. In some cases in our test suite, we are referencing a common REST Request TestCase, returning the response as a custom property on that common TestCase, and running an Assertion TestStep on that response property from within many different TestCase's. When I try to add a Script Assertion against that common REST Request TestCase, I am unable to because the 'Script' option on the 'Assertions' list is greyed out and disabled (screenshot below). So in order to make this work I will need to be able to either:

1) Assert an empty value from the standard Message Content Assertion assertion type without getting a 'Node Missing' message.

2) Select the 'Script' option when asserting against a property that is returned from a 'Run TestCase' TestStep.