[B] Allow null modifier functions in EntityDamageEvent. Fixes Bukkit-5688

<b>The Issue:</b>

Currently, damaging a non-living entity (such as an item frame) throws an NPE and crashes the server.

<b>Justification for this PR</b>

This pull request fixes a huge internal server error caused by Bukkit.

<b>PR Breakdown:</b>

Checks for a null value of modifierFunctions in EntityDamageEvent, and refrains from accessing the variable to prevent an NPE.

<b>JIRA Ticket:</b>

BUKKIT-5688 - https://bukkit.atlassian.net/browse/BUKKIT-5688