Physics: Fix a regression from the multithreading refactoring that ma…

…de the movement tracker panic when playing remote, because the player wasn't loaded yet at the time the first physics tick was made (the map isn't loaded either).

src/networking/movement_tracker.rs

@@ -48,7 +48,10 @@ impl MovementTracker {
             .upgrade()
             .expect("World Server to outlive Movement Tracker");
 
-        let player_guid = world.player_guid.get().expect("Player Guid is already set");
+        let Some(player_guid) = world.player_guid.get() else {
+            return; // The physics thread is started before we're even properly loaded and connected.
+        };
+
         let timestamp = world.get_timestamp();
 
         let info = Self::build_movement_info(delta_unrotated, absolute_position, orientation, timestamp);