edit predictions: Handle no keybind for preview (#25015)

If the user removes all modified keybinds in `edit_prediction_conflict`,
the preview bar above the completions menu would disappear. This PR
handles that case slightly better by still showing the 1-line preview
(which they might accept via an unmodified keybind) and hides the `|
Preview ⌥` section since it's impossible to invoke in this case.

Release Notes:

- Handle `edit_prediction_conflict` context without modified keybinds
for `AcceptEditPrediction`