MultiplayerSpawner spawn() incorrectly defines parameter

[DMillerF]

Your Godot version: 4.3

Issue description: The defined function spawn for MultiplayerSpawner lists the parameters as (data: Variant = null). However, this parameter is not optional, and a default value will never be used. It is unclear if the parameter is intended to be optional, but current implementation results in it being a requirement for any Callable assigned to spawn_function

Failing to provide a value for data results in the following misleading error:

URL to the documentation page: https://docs.godotengine.org/en/4.3/classes/class_multiplayerspawner.html

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