Fixed bug when checking email batch size

- even though we don't do anything yet, the code was calling `.length`
  on an object, which is `undefined`
- this fixes that by checking the length of the keys
- also moves the code block down so we can early-return if mailgun is
  not configured

ghost/mailgun-client/lib/mailgun-client.js

@@ -31,16 +31,16 @@ module.exports = class MailgunClient {
      * }
      */
     send(message, recipientData, replacements) {
-        if (recipientData.length > module.exports.BATCH_SIZE) {
-            // err - too many recipients
-        }
-
         const mailgunInstance = this.getInstance();
         if (!mailgunInstance) {
             logging.warn(`Mailgun is not configured`);
             return;
         }
 
+        if (Object.keys(recipientData).length > module.exports.BATCH_SIZE) {
+            // TODO: what to do here?
+        }
+
         let messageData = {};
 
         try {