Improve error message for NO_OUTPUT in case of empty string

src/test/TestBase.java

@@ -240,7 +240,10 @@ public abstract class TestBase {
                     if (output != null) {
                         log.info(" Failed!\n");
                         printLog();
-                        fail("Output should have been empty but was:" + output);
+                        if (output.equals("")) {
+                            output = "(empty string)";
+                        }
+                        fail("Output should have been empty but was: " + output);
                     }
                     break;
                 case NO_CHECK: