GDScript custom function argument hint correction for Variant and Nil types

Created by: dalexeev

… types

Custom functions now have the same argument hint format as the built-in functions. Continuation of the #35199 PR.

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https://github.com/godotengine/godot/blob/f2aa99a8e2d33e534fa3adbff2981a9c902bbf32/modules/gdscript/gdscript_parser.cpp#L5479

It could have been possible to change this line, but I was not sure that it would not affect other parts of the code.