Add empty content when history is empty

[julien-nc]

closes #62 The history task list is retrieved after the history button is clicked so we can't hide the button if the history is empty. But we can show an empty content:

[julien-nc]

Since it's possible to change the task type while seeing the history, maybe it's still good to see the history button even if it leads to an empty history.

It might seem weird to users not to find the history button.