LightUpdate: account for virtual nodes in prepareNodes()

fixes #3959

src/world/light/LightUpdate.php

@@ -114,6 +114,9 @@ abstract class LightUpdate{
 						$context->removalQueue->enqueue([$x, $y, $z, $oldLevel]);
 					}
 				}
+			}elseif($this->getEffectiveLight($x, $y, $z) > 0){ //outside the chunk (e.g. virtual sky light from y=256)
+				$context->spreadVisited[$blockHash] = true;
+				$context->spreadQueue->enqueue([$x, $y, $z]);
 			}
 		}
 		return $context;