Fixes #1458 | Implement null check for LocalPlayer in renderLevel

[Rodriggrr]

Problem

GameRendererMixin.renderLevel dereferences this.minecraft.player without a
null check. During the window between connecting to a server and the player
entity being fully assigned, renderLevel still runs and throws an NPE,
crashing the client. This is 100% reproducible on multiplayer connect but
doesn't show up in singleplayer, matching the reports in #1458 and #107.

Fix

Add a null check at the top of the injected method; reset sable$lastSubLevel
and return early if player is null, matching the existing behavior in the
else branch.

Fixes #1458

[CLAassistant]

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