Use a LogicalEquals functoid to determine if your input matches your case. Then map that output to a LogicalNot. Send the output of the LogicalEquals to one Mapping functoid (second input for Mapping will be the value of your field) and the output of your LogicalNot to another (same second input). Then map both of the value mapping functoids to your destination node.
Then, when your value equals your case, you'll send FALSE to the destination (instead of Null or Empty) and the node won't be output. Any other time, the LogicalNot will fire its Mapping Functoid and you should get your desired output.