Confusing error message when using an empty object for a decorator.

### **feature request**

### What is the current behavior?
If you pass in an empty object into an array of decorators, you get an error that is difficult to track down. I encountered this error because I had a placeholder decorator that was just an empty object.

Here is the error message:

### What is the expected behavior?
A handled error that states decorators must have at least one key/field.

### Sandbox Link

### What's your environment?
react-final-form version: ^6.5.0
final-form version: ^4.20.0
macOS Mojave version: 10.14.6
node version: 10.16.3

### Other information