Fix NullPointerException in LogicBlock.updateLinks() when linksVar is null

[ZenthXSin]

Bug Fix

Problem:
LogicBlock.LogicBuild.updateLinks() crashes with NullPointerException when linksVar is null.

java.lang.NullPointerException: Cannot assign field "numval" because "this.linksVar" is null
    at mindustry.world.blocks.logic.LogicBlock$LogicBuild.updateLinks(LogicBlock.java:610)
    at mindustry.world.blocks.logic.LogicBlock$LogicBuild.updateTile(LogicBlock.java:567)

Root Cause:
linksVar is annotated as @Nullable (line 266) and only initialized during code compilation in buildLASM() (line 392). However, updateLinks() (line 575) directly accesses linksVar.numval without null checking. This can happen when:

• A logic block just spawned/placed and code hasn't compiled yet
• Code compilation failed (exception caught, empty code loaded)
• Link state changes before code is loaded

Fix:
Added null check at the beginning of updateLinks():

public void updateLinks(){
    if(linksVar == null) return;
    ...
}

This prevents the NPE and gracefully handles the case where links update before code compilation.

[cardillan]

Just FYI, #12232 also fixes this (in the same way).