Do not show a dialog about changing formats when there is no content

1. Go to node/add/article to add an article:
* The default format is Basic HTML
* The editing area is empty
2. Without adding anything inside CKEditor, switch text format to Full HTML
3. The alert message is shown:

"Changing the text format to Full HTML will permanently remove content that is not allowed in that text format.
Save your changes before switching the text format to avoid losing data."

Note that there is no content to be saved and there is nothing that can be destroyed at this moment. An unaware user will be surprised and confused. The alert message could be shown only when there is some content inside editor while switching input formats.