fix: Don't throw if drag surface is empty. (#5695)

Observed this error in Blockly Games Bird 6 (Chrome OS):
Uncaught TypeError: Failed to execute 'appendChild' on 'Node': parameter 1 is not of type 'Node'.
It has only shown up once, and I can't reproduce, but it looks like the drag surface was being cleared while empty.  Maybe some weird multi-touch operation?

Open question: Leave the code as is so that the error is thown and visible, or supress it?

core/block_drag_surface.js

@@ -252,11 +252,14 @@ BlockDragSurfaceSvg.prototype.getWsTranslation = function() {
  * being moved to a different surface.
  */
 BlockDragSurfaceSvg.prototype.clearAndHide = function(opt_newSurface) {
- if (opt_newSurface) {
- // appendChild removes the node from this.dragGroup_
- opt_newSurface.appendChild(this.getCurrentBlock());
- } else {
- this.dragGroup_.removeChild(this.getCurrentBlock());
+ const currentBlockElement = this.getCurrentBlock();
+ if (currentBlockElement) {
+ if (opt_newSurface) {
+ // appendChild removes the node from this.dragGroup_
+ opt_newSurface.appendChild(currentBlockElement);
+ } else {
+ this.dragGroup_.removeChild(currentBlockElement);
+ }
  }
  this.SVG_.style.display = 'none';
  if (this.dragGroup_.childNodes.length) {