While working on a Soul Forge command block module a while back, I had need for an empty spawner (so that players could create one). I achieved this by creating a spawner with the following BlockEntityTag:
{SpawnData:

,SpawnPotentials:[{Weight:1,Entity:{id:"null"}}],Delay:400s}
(I know that Minecraft doesn't serialize nulls, but I needed to put a nonempty string).
Several snapshots back, when placed, that spawner was indeed visually empty, and when right-clicked with a spawn egg, would change to a spawner of the egg type correctly. However, in the most recent snapshot, when placed and right-clicked, ONLY SpawnData is changed. SpawnPotentials remains with the null entity, and after a spawn cycle resets the spawner back to fully null.
This behavior is not seen with valid entity types in SpawnData, only invalid ones. Unfortunately, invalid types are useful as placeholders, so please do fix this, as it used to work fine.