Bug 1683548: Fix sending messages containing inline images. r=mkmelin

Differential Revision: https://phabricator.services.mozilla.com/D100219

--HG--
extra : amend_source : b898b49f598f6c6dc05c3bd877192818bb501a42

mailnews/compose/src/MimeMessageUtils.jsm

@@ -1018,6 +1018,7 @@ var MsgUtils = {
       if (matches && matches[1]) {
         return matches[1];
       }
+      return "";
     }
     // Take the part after the last / or \.
     let lastSlash = url.lastIndexOf("\\");