Otherwise some champs are created before the previous one has been
saved, and React complains that several champs have the same
`champ-undefined` key.
(Plus it made the tests flaky and unreliable.)
Before the editor attempted to create a default champ as soon as the
list became empty.
This created many race conditions, which made the tests flaky.
Remove this behavior, and add an empty label instead.
Before, when the "Add new champ" button was clicked, the new champ
was inserted after the **first** fully visible champ.
That was most of the time unexpected. The correct behavior would be to
insert the new champ after the **last** fully visible champ.
That's what this commit does. Now the "Add new champ" behavior feels
much less confusing.
