Fix ExprDifference

[TPGamesNL]

Description

Fix issues with ExprDifference.

Test script used: https://pastebin.com/BhZKtvMr

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Target Minecraft Versions: any
Requirements: none
Related Issues: #661, #575, #994, #1001, #2302

[ShaneBeee]

Maybe it would be a good idea to add a regression test for this, to make sure all cases work and this doesn't break anything else.

[TPGamesNL]

So I tried some tests:

test "difference expression with parse expression":
	set {_a} to 3
	set {_b} to 5
	assert difference between {_a} and {_b} is 2 with "difference expr with a=var and b=var failed"
	assert difference between {_a} and "%{_b}%" parsed as a number is 2 with "difference expr with a=var and b=parse failed"
	assert difference between "%{_a}%" parsed as a number and {_b} is 2 with "difference expr with a=parse and b=var failed"
	assert difference between "%{_a}%" parsed as a number and "%{_b}%" parsed as a number is 2 with "difference expr with a=parse and b=parse failed"

and, because I wasn't running it on my fix, got a console error: https://pastebin.com/bH77tMnc. The problem is, after it threw this error, testing stopped completely. I suggest catching exceptions in some places where the tests can lead to exceptions.

EDIT: I'd also like to draw your attention to https://github.com/SkriptLang/Skript/runs/1701328945?check_suite_focus=true#step:7:144, not sure if it's been reported or not

The base branch was changed.