A resourcepack without a pack image shows the default resource pack image instead of unknown_pack.png
Including resourcepack to have you see. The resource pack have no pack.png in it.

Steps to reproduce:
(You want to download or make the resource pack first.)
1. Install resource pack at %appdata%/.minecraft/resourcepacks
2. Open Minecraft (or atlease refresh the resource packs page)
3. See the bug happen