[core] Do something about being able to display empty lines in formatted sad::Label

[mamontov-cpp]

See example

<div bold="true">This is game</div> 
<div> </div>
<div></div>
<div>    </div>
<div></div>
<div underline="true">Game is shit</div>

Note, that there is at least six lines. But we can see two.

[mamontov-cpp]

Note, that this bug exists only in formatted labels. It does not exists in non-formatted labels.