perf: fast exit for empty string (#790)

Signed-off-by: francesco <francesco.bagnoli.69@gmail.com>

Author: cesco69

lib/serializer.js

@@ -89,7 +89,9 @@ module.exports = class Serializer {
 
   asString (str) {
     const len = str.length
-    if (len < 42) {
+    if (len === 0) {
+      return '""'
+    } else if (len < 42) {
       // magically escape strings for json
       // relying on their charCodeAt
       // everything below 32 needs JSON.stringify()