'A' tags in Custom Blocks get removed
Just tested this on a fresh install of 8.0.6 and I think I see what is going on. 

I saved a block using the Full HTML format with the following content:

&lt;a href="#"&gt;&amp;nbsp;&lt;/a&gt;
&lt;a href="#"&gt;&lt;/a&gt;


And had it display in my sidebar region. Everything worked fine, there are two links in this block, that can only really be seen if you inspect the html.

However, when going to edit the block again, with the default as-installed text format settings, ckeditor will wrap my first link with a P tag, and completely remove the second link. 

So, you have a few options.


Enter an &amp;nbsp; character inside your empty tag.
Disable the ckeditor for your format and just use the plain-text editor, with the Full HTML (or a custom filter of your creation) filter.


Ckeditor is doing the fixing of your html, so getting rid of it will solve your problem.