Improve handling of empty-label links

[neiljp]

Currently the following cause ~empty messages: (there may be other cases to add)

• [](https://www.camera-rumors.com/wp-content/uploads/2018/02/sony-a7iii-sample-image-11.jpg)
• [](google.com)

UPDATE 2023-11:
It appears that a regular URL works OK (second case) since this is handled by the server - at least at this time/version.

The first case appears to still be a problem. Based on inspecting the browser content (which may be different) this may be related to this resulting in something like the expected <A HREF="..."><IMG></A> but with a server-relative URL for the image starting with /external_content/.