Fix function not being called

[TPGamesNL]

Description

This PR includes:

• A fix for Functions not working when passing empty list variable #3687 by reverting Prevent Invalid Function Parameter Type from AIOOB #2495:
  • Prevent Invalid Function Parameter Type from AIOOB #2495 was meant to fix an issue invalid types being used in Java functions (Functions don't work properly #2384), by preventing any function (not just Java functions) being called with null as the first parameter.
• Functions don't work properly #2384 is also fixed in this PR, this was caused by Java functions not checking for null values. I have created a class called SimpleJavaFunction which checks if any given arguments are null for you, so this didn't have to be done for every Java function (the class also removes the FunctionEvent as a parameter for the execute method, because it was never used)
• I've created a utility class called DefaultClasses, containing references to common ClassInfos, so Classes.getExactClassInfo(Class) doesn't need to be called everywhere in Java function registration (partly because it's not needed, partly because getExactClassInfo is nullable.
• I've fixed some formatting in function classes, such as:
  • Removing the final modifier from variables and parameters
  • Removing tautological assert statements
  • Doing proper indentation and bracketing
• A fix for Unexpected behavior on Skript reload #3660, first implemented in Update function references when the function definition is reloaded #3795 (but improved in this PR)

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Target Minecraft Versions: any
Requirements: none
Related Issues: #3687, #2384, #3660