[Truffle] Real implementation of String#empty?.

Author: nirvdrum

truffle/src/main/java/org/jruby/truffle/nodes/core/StringNodes.java

@@ -768,9 +768,7 @@ public EmptyNode(EmptyNode prev) {
 
         @Specialization
         public boolean empty(RubyString string) {
-            notDesignedForCompilation();
-
-            return string.toString().isEmpty();
+            return string.getBytes().length() == 0;
         }
     }