Issue title: Drag & drop: content disappears when dragged from restricted editing area in Safari
 Issue body: Extracted from: https://github.com/ckeditor/ckeditor5/pull/9297#issuecomment-802634131

## 📝 Provide detailed reproduction steps (if any)

1. Go to restricted editing docs ( checked on latest `release` )
2. Turn on restricted editing mode
3. Select some content inside restricted edititing area and drag it somewhere

### ✔️ Expected result

Nothing happens - content stays at its original position.

### ❌ Actual result
Content disappears:  

https://user-images.githubusercontent.com/34380544/111806416-3cd47b80-88d2-11eb-9e54-a56e221a55b2.mp4



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 Comments: 
 Comment 0: Does not seem to be very likely to happen in real-life scenarios. Also, one can undo from this.
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