Fix changing to <none> not functional

[TPGamesNL]

Description

Changes EffChange so that setting one value to another, where the second value evaluates to null (<none>), actually deletes the first value instead of not having any effect (I asked on in skript-talk in Discord if this should be the expected behaviour, this was the result).

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Target Minecraft Versions: any
Requirements: none
Related Issues: #3049 (partial)

[yhdev7935]

I wrote the code using these grammatical characteristics, but after this commit, everything needs to be fixed. That's sad. 😢
I used this grammar when I set the default value.
However, this commit seems to prevent many unexpected errors. ( Especially in code that uses loop frequently. )

command /test [<text>]:
  trigger:
    set {_arg1} to "Hi" # The code length is shorter than "if arg 1 is not set" :D
    set {_arg1} to arg-1

[FranKusmiruk]

I wrote the code using these grammatical characteristics, but after this commit, everything needs to be fixed. That's sad. 😢
I used this grammar when I set the default value.
However, this commit seems to prevent many unexpected errors. ( Especially in code that uses loop frequently. )

command /test [<text>]:
  trigger:
    set {_arg1} to "Hi" # The code length is shorter than "if arg 1 is not set" :D
    set {_arg1} to arg-1

I am honestly surprised someone managed to take advantage of this, however it is broken behaviour and it should be fixed accordingly.

[TPGamesNL]

@yhdev7935 Also take a look at the default value expression: https://skriptlang.github.io/Skript/expressions.html#ExprDefaultValue

[yhdev7935]

@TPGamesNL Oh, Thanks. However, the difficulty of debugging will inevitably increase slightly.