EntityUpdate-style endpoints behave differently from String and List<EntityUpdate>

[andrus]

From these 3 styles of update endpoints:

public SimpleResponse sync(String requestBody)
public SimpleResponse sync(EntityUpdate<T> u)
public SimpleResponse sync(List<EntityUpdate<T>> us)

The first and third variants treat an empty payload as an empty list of updates. While the second one fails with 400 No update. We need to align the behavior between the styles. I think treating this input as valid ("empty list of updates") is the way to go.