Update comment to new placeholder system

src/edu/kit/informatik/Terminal.java

@@ -148,7 +148,7 @@ public final class Terminal {
                 throw new RuntimeException(e);
             }
         else {
-            //If the program outputted nothing we still have at least an empty line
+            //If the program outputted nothing we still have null which means "no output"
             OUT_TEST.add(null);
 
             assert !IN_TEST.isEmpty()