[LUA] Fix Tenshodo Coffer uses KI when reward is unobtainable

[ampitere]

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• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Fixes an issue with the Tenshodo Coffer where the KI will be consumed even if a player cannot receive the current reward item. player:addItem returns whether or not it was successful so we just need to flip the if..then logic and add it there. Resolves #4727

Steps to test these changes

1. !pos 41.169 3.899 -51.005 245
2. !addkeyitem 1184
3. !additem 4751
4. Open until you get a Scroll of Erase or your inventory is full