MultiplayerSpawner spawn() incorrectly defines parameter

**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

If you know how to fix the issue you are reporting please
consider opening a pull request. We provide a tutorial on
using git here: https://docs.godotengine.org/en/stable/community/contributing/pr_workflow.html,
writing documentation at https://docs.godotengine.org/en/stable/community/contributing/docs_writing_guidelines.html
and contributing to the class reference here: https://docs.godotengine.org/en/stable/community/contributing/updating_the_class_reference.html

Start with the MultiplayerSpawner class reference at the linked documentation page and inspect the documented spawn(data: Variant = null) signature. Compare the documented parameter behavior with the reported spawn_function Callable requirement, then update the class reference so the signature and requirement are accurate.

Written by the indexing model from the issue text.