Fix a very rare light propagation bug

This could occur when light sources were placed near each other and one of them is removed, the other's light would not be propagated far enough because the node wasn't queued for updating although it was marked as if it was.

src/pocketmine/level/light/LightUpdate.php

@@ -158,11 +158,9 @@ protected function computeSpreadLight(int $x, int $y, int $z, int $newAdjacentLe
 		if($current < $potentialLight){
 			$this->setLight($x, $y, $z, $potentialLight);
 
-			if(!isset($this->spreadVisited[$index = Level::blockHash($x, $y, $z)])){
+			if(!isset($this->spreadVisited[$index = Level::blockHash($x, $y, $z)]) and $potentialLight > 1){
 				$this->spreadVisited[$index] = true;
-				if($potentialLight > 1){
-					$this->spreadQueue->enqueue([$x, $y, $z]);
-				}
+				$this->spreadQueue->enqueue([$x, $y, $z]);
 			}
 		}
 	}