field: change signature of group raise_event

[salix5]

void raise_event(card* event_card, uint32 event_code, effect* reason_effect, uint32 reason, uint8 reason_player, uint8 event_player, uint32 event_value);
void raise_event(const card_set& event_cards, uint32 event_code, effect* reason_effect, uint32 reason, uint8 reason_player, uint8 event_player, uint32 event_value);

Now group version of raise_event use a const reference.
The overload resolution will not fail when the first parameter is nullptr.

group版本的raise_event改成使用const reference
現在第一個參數傳入nullptr時2個重載版本不再會混淆

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