fix buglet in Zend_Json::prettyPrint

git-svn-id: http://framework.zend.com/svn/framework/standard/trunk@19910 44c647ce-9c0f-0410-b52a-842ac1e357ba

library/Zend/Json.php

@@ -363,7 +363,7 @@ class Zend_Json
             $prefix = str_repeat($ind, $indent);
             if($token == "{" || $token == "[") {
                 $indent++;
-                if($result[strlen($result)-1] == "\n") {
+                if($result != "" && $result[strlen($result)-1] == "\n") {
                     $result .= $prefix;
                 }
                 $result .= "$token\n";