Block filler must be rendered in empty lines created by <br>

[Reinmar]

Unfortunately, we must achieve this when rendering a block which ends with a line break:

<p>...<br>BLOCK_FILLER</p>

BLOCK_FILLER is a <br data-cke-filler=true> inside the editing view and   in the data.

So, we need to adjust the logic which decides where to place block fillers. In general, block filler is needed after a <br> if that <br> is the last node of a container element:

<p>XXX<br>YYY<br>ZZZ<br>BLOCK_FILLER</p>
<p>XXX<br>YYY<br>ZZZ<br>ĄŚĆ</p>
<p><br>YYY<br>ZZZ<br>ĄŚĆ</p>