fix: null-safe upgradeDateTime conversion in BuildingManager.setUpgradeDateTime

Placing a banestone (and other callers) pass null to clear a fresh
building's upgrade timer, but the inline .atZone(...).toInstant() NPE'd
on null. Convert only when non-null; the DB layer already handles null.

src/engine/gameManager/BuildingManager.java

@@ -25,6 +25,7 @@ import engine.objects.*;
 import engine.server.MBServerStatics;
 import org.pmw.tinylog.Logger;
 
+import java.time.Instant;
 import java.time.LocalDateTime;
 import java.time.ZoneId;
 import java.util.ArrayList;
@@ -274,7 +275,8 @@ public enum BuildingManager {
 
         if (building == null)
             return;
-        if (!DbManager.BuildingQueries.updateBuildingUpgradeTime(upgradeDateTime.atZone(ZoneId.systemDefault()).toInstant(), building, rankCost)) {
+        Instant instant = (upgradeDateTime == null) ? null : upgradeDateTime.atZone(ZoneId.systemDefault()).toInstant();
+        if (!DbManager.BuildingQueries.updateBuildingUpgradeTime(instant, building, rankCost)) {
             Logger.error("Failed to set upgradeTime for building " + building.getObjectUUID());
             return;
         }