What I expected to happen was...:
Display the item tooltip for a block of air when mousing over it, or simply display no tooltip at all.

What actually happened was...:
The game crashed

Steps to Reproduce:
1. Launch Minecraft in creative mode with cheats turned on
2. Use the /give command to obtain a command block (/give @p 137)
3. Place the command block and enter the following command: "/summon Villager ~ ~1 ~ {Offers:{Recipes:[{buy:

,sell:

,maxUses:10,uses:0}]}}" (without the quotation marks of course)
4. Activate the command block, spawning a villager
5. Open the trading UI with the villager and mouse over where the offered items of the trade are supposed to be located.

I was hoping to use the command to spawn villagers with blank trades for an adventure map, but my hopes were quickly crushed when my game crashed.