Formatting text can remove empty lines

[rangar]

If there are two <br /> positioned before unformatted text, and the text is changed to <h2>, the editor removes both <br />. Seen from the perspective of the user, formatting the text removes an empty line, which is probably unexpected.

[Jakub Ś]

Problem can be reproduced from CKEditor 4.0 in all browsers.

To reproduce:

1. Set editor to enter mode BR and clear its contents
2. Press Enter 3 times
3. Type few characters
4. Select H2 format

Result: One BR/line is removed.

NOTE: It is possible that this ticket is related to #11392.