BSB produces empty <header></header> tags when block title is empty

Steps to reproduce

1. Enable Borealis Semantic Blocks.
2. Create a block with an empty title.

What happens

BSB produces an empty pair, e. g.:

<nav class="block block-menu" id="block-menu-menu-documents">
  <header>  </header>
  <ul class="menu">
    <li class="first leaf">...</li>
    ...
  </ul>
</nav>

What should happen

The tag pair should be omitted, e. g.:

<nav class="block block-menu" id="block-menu-menu-documents">
  <ul class="menu">
    <li class="first leaf">...</li>
    ...
  </ul>
</nav>