AI: comp use boost cards on defend stage without attackers

Computer cast boost card (+3/+3 by [Crash the Ramparts](https://scryfall.com/card/xln/182)) with no threat on defend stage (no one attacks it):

```
5:27: Turn 6 JayDi (20 - 20)
5:27: JayDi draws a card
5:28: JayDi sacrificed Treasure [ee9]
5:28: JayDi casts Headstrong Brute [185]
5:28: JayDi puts Headstrong Brute [185] from stack onto the Battlefield
5:28: JayDi attacks with 0 creatures
5:28: computer casts Crash the Ramparts [872] targeting Drover of the Mighty [c6a]
5:28: computer puts Crash the Ramparts [872] from stack into his or her graveyard
```