fix: handle missing attachments in Outlook message conversion

Add fallback to empty array when iterating attachments in
convertMessageToUploadObject to prevent TypeError if attachments
key exists but value is null/undefined.

Author: andris9

lib/email-client/outlook-client.js

@@ -2015,7 +2015,7 @@ class OutlookClient extends BaseClient {
                 case 'attachments': {
                     messageUploadObj.attachments = [];
                     let attachmentCounter = 0;
-                    for (let attachment of emailObject.attachments) {
+                    for (let attachment of emailObject.attachments || []) {
                         let attachmentEntry = {
                             '@odata.type': '#microsoft.graph.fileAttachment'
                         };