Remove Nashorn-specific comment

I couldn't reproduce a case of Nashorn returning null for a non-existent
property, of either a Java object or a JavaScript object, even going
back to Java 8u20. But the test makes sense in any case, so I just
removed the mention of Nashorn.

Author: jonathanperret

src/main/js/lib/tabcomplete.js

@@ -183,11 +183,7 @@ var onTabCompleteJS = function() {
           symbol = null;
           break;
         }
-        if (typeof symbol == 'undefined') {
-          break;
-        }
-        // nashorn - object[missingProperty] returns null not undefined
-        if (symbol == null) {
+        if (typeof symbol == 'undefined' || symbol === null) {
           break;
         }
         lastGoodSymbol = symbol;