Bail out if there are no files to upload.

Without this change, if you either:
  - Click and drag some text into the markdown editor, or
  - Copy paste some text into the markdown editor

The status bar will get updated to say "Uploading image ...", and will
never stop.

src/js/easymde.js

@@ -1669,6 +1669,9 @@ function EasyMDE(options) {
  * @param [onError] {function} see EasyMDE.prototype.uploadImage
  */
 EasyMDE.prototype.uploadImages = function (files, onSuccess, onError) {
+    if (files.length === 0) {
+      return;
+    }
     var names = [];
     for (var i = 0; i < files.length; i++) {
         names.push(files[i].name);
@@ -1688,6 +1691,9 @@ EasyMDE.prototype.uploadImages = function (files, onSuccess, onError) {
  * @param {FileList} files The files to upload the the server.
  */
 EasyMDE.prototype.uploadImagesUsingCustomFunction = function (imageUploadFunction, files) {
+    if (files.length === 0) {
+      return;
+    }
     var names = [];
     for (var i = 0; i < files.length; i++) {
         names.push(files[i].name);