Set null contents to null instead of error throw

Before, if a cookie didn't exist, it would throw an error, not causing permissions/tokens to change to empty values. This fixes that (and #626) by setting the value to undefined instead.

src/lib/jar.js

@@ -15,7 +15,8 @@ var Jar = {
         // Return the usable content portion of a signed, compressed cookie generated by
         // Django's signing module
         // https://github.com/django/django/blob/stable/1.8.x/django/core/signing.py
-        if (!value) return callback('No value to unsign');
+        if (typeof value === 'undefined') return callback(null, value);
+        
         try {
             var b64Data = value.split(':')[0];
             var decompress = false;
@@ -80,6 +81,8 @@ var Jar = {
             if (err) return callback(err);
             Jar.unsign(value, function (err, contents) {
                 if (err) return callback(err);
+                if (typeof contents === 'undefined') return callback(null, contents);
+                
                 try {
                     var data = JSON.parse(contents);
                 } catch (err) {