fix(ai): Cast spell when all enemies are invisible

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a31d510) 96.21% compared to head (4ef7671) 96.22%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #306 +/- ## ========================================= Coverage 96.21% 96.22% - Complexity 8634 8635 +1 ========================================= Files 1358 1358 Lines 21913 21915 +2 Branches 1318 1318 ========================================= + Hits 21084 21087 +3 Misses 638 638 + Partials 191 190 -1 ``` | [Files Changed](https://app.codecov.io/gh/Arakne/Araknemu/pull/306?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Arakne) | Coverage Δ | | |---|---|---| | [...trevieux/araknemu/game/fight/ai/action/Attack.java](https://app.codecov.io/gh/Arakne/Araknemu/pull/306?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Arakne#diff-c3JjL21haW4vamF2YS9mci9xdWF0cmV2aWV1eC9hcmFrbmVtdS9nYW1lL2ZpZ2h0L2FpL2FjdGlvbi9BdHRhY2suamF2YQ==) | `97.67% <100.00%> (+0.11%)` | :arrow_up: | | [.../araknemu/game/fight/ai/action/util/CastSpell.java](https://app.codecov.io/gh/Arakne/Araknemu/pull/306?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Arakne#diff-c3JjL21haW4vamF2YS9mci9xdWF0cmV2aWV1eC9hcmFrbmVtdS9nYW1lL2ZpZ2h0L2FpL2FjdGlvbi91dGlsL0Nhc3RTcGVsbC5qYXZh) | `100.00% <100.00%> (+7.14%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/Arakne/Araknemu/pull/306/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Arakne)

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