📝 Provide detailed reproduction steps (if any)

This is an issue similar to recently closed #18960 and leads to the same errors.

1. Create an empty Google Docs document.
2. In Google Docs document, press "Tab" key, then write "Foo bar".
3. Copy the contents of the document.
4. Open CKE5 manual test (any should do).
5. Paste.
6. Call editor.getData(). You will get <p>\tFoo bar</p>.
7. Call editor.setData( editor.getData() ).
8. Then call editor.getData(). You will get <p>Foo bar</p>.
9. \t was silently removed.

This causes crashes in revision history plugin.

Most probably end of a block element is also affected, and possibly other whitespace characters may lead to the same problem.