[core] nullptr check for Rune Enhancement

[Omnione]

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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?

Adds a nullptr check for weapons in battleutils.cpp for Rune Enhancment, to prevent crashes when entities other than player characters have en dmg calculated.

Steps to test these changes

Call Amachuchu to fight, engage a mob, she should use runes and do en dmg but not crash the server.