[DMN Designer] The `defaultOutputValue` and the `outputValue` are being persisted when the value is empty in the UI

When the user persists the following diagram:

..with the following Decision Table:

..select the "Can drink beer?" header, and set no output:

..the persisted value will be something like this:

<dmn:output id="_E9E45B42-9506-4D66-BAAF-44B161590B43" name="output-1" typeRef="string">
<dmn:outputValues id="_F02DC5E5-ECCE-4B3A-87A8-9CDC74A7F81A">
<dmn:text></dmn:text>
</dmn:outputValues>
<dmn:defaultOutputEntry id="_A7CFCB22-3D05-4CDB-925B-D4FD52F3D018" typeRef="string">
<dmn:text></dmn:text>
</dmn:defaultOutputEntry>
</dmn:output>

..and the expected behaviour is to not have the tags `dmn:outputValues` and `dmn:defaultOutputEntry` when the value for them is empty, like this:

<dmn:output id="_E9E45B42-9506-4D66-BAAF-44B161590B43" name="output-1" typeRef="string">
</dmn:output>