Cleanup method handle utils & fix npe

[Pablete1234]

Fixes an NPE that may occur when a player not in the match does certain events that are matched by a dynamic filter:

[MatchImpl] Unable to invoke cached MethodHandle extracting Filterable for org.bukkit.event.player.PlayerItemHeldEvent@4f28c252
java.lang.NullPointerException
 	at java.base/java.util.Objects.requireNonNull(Objects.java:221)
 	at tc.oc.pgm.filters.FilterMatchModule.invalidate(FilterMatchModule.java:427)
 	at tc.oc.pgm.filters.FilterMatchModule.lambda$registerListenersFor$17(FilterMatchModule.java:469)

This rooted from the PlayerItemHeldEvent having a player, but match.getPlayer(bukkitPlayer) returning null (player not properly in the match) which caused the npe when trying to invalidate.

Also does a bit of a cleanup of method handle finding