A normal custom tags in docbook looks like this:

<eztemplate name="vcustom" ezxhtml:class="ez-custom-tag ez-custom-tag--content-visible">
       <ezcontent>
           <para>custom tag content</para>
       </ezcontent>
       <ezconfig>
          (...)
       </ezconfig>
   </eztemplate>

The ezxmltext->docbook converter will (if custom tag has no "content") leave out the <ezcontent>...</ezcontent> tag...
This is fine according to RNG schema...

However, if you in twig template for that customtag has {{ content | raw }} you'll get fatal error because variable is not set

Also, if you in admin-ui creates a inline custom tag with empty content, that tag will also have no <ezcontent/> tag and you'll get the same fatal when rendering the template

Reading the docs, you would expect the content variable to always exists...