dont display empty prompt buttons

[NBKelly]

This isn't a great fix, because I still have no clue how empty options end up in the prompt state, but at the very least this is a stopgap.

Honestly, if somebody else can spy a reason for there to be some extra entries in the prompt state, they're better at inspecting code than I am 😂

It adds a test, but that test was just to determine that the issue was actually on the frontend and not the backend (still, doesn't hurt).

Closes #7424

[NoahTheDuke]

yeah something fucky is happening here. i suspect it's in trigger-queued-event-player:

          choices-map (map #(vector (or (:ability-name (:ability (:handler %)))
                                        (card-for-ability state (:handler %)))
                                    %) cards-with-titles)
          choices-titles (map first choices-map)

this is where a nil value will leak in. i'm not sure why, but maybe we should use keep? that'll remove any cards-with-titles that aren't allowed (idk why card-for-ability fails only here and not earlier).

[NoahTheDuke]

jk i figured it out, it's a real dumb issue lmao, it's tied to #7564.

[NoahTheDuke]

Covered by #7565