If you were to enter "/setblock ~ ~ ~ spawner{SpawnData:{entity:{id:"cow"}},SpawnPotentials:[{Entity:

,Weight:1},{Entity:

,Weight:2}]}" you would find that it only spawns cows.
If you get the data of the block, you see that the SpawnPotentials tag is present, but empty. It is supposed to default to the entity from the SpawnData tag if it is empty, but it remains completely empty. If the SpawnPotentials tag is not specified it is supposed to default to a pig, but it is still empty.