Document pick_random for empty arrays

[novalis]

No description provided.

[Sauermann]

Executing pick_random on an empty array results in the following error message:

E 0:00:02:0415   Control.gd:64 @ _on_button_2_pressed(): Can't take value from empty array.
  <C++ Error>    Condition "_p->array.size() == 0" is true. Returning: Variant()
  <C++ Source>   core/variant/array.cpp:329 @ pick_random()
  <Stack Trace>  Control.gd:64 @ _on_button_2_pressed()

So I'm not entirely convinced, that documenting a behavior which causes an error message is warranted.

[AThousandShips]

We can possibly document that it requires a non empty array, though that should be obvious, as it says "from"

[Sauermann]

Sounds good. The documentation of Array.back and Array.front could be used for that.

[AThousandShips]

LGTM, clear and concise

[akien-mga]

Could you squash the commits? See PR workflow for instructions.

[akien-mga]

Thanks!