Inventory: added API method isSlotEmpty()

src/pocketmine/inventory/BaseInventory.php

@@ -237,6 +237,10 @@ abstract class BaseInventory implements Inventory{
 		return -1;
 	}
 
+	public function isSlotEmpty(int $index) : bool{
+		return $this->slots[$index] === null or $this->slots[$index]->isNull();
+	}
+
 	public function canAddItem(Item $item) : bool{
 		$item = clone $item;
 		$checkDamage = !$item->hasAnyDamageValue();

src/pocketmine/inventory/Inventory.php

@@ -171,6 +171,15 @@ interface Inventory{
 	 */
 	public function firstEmpty() : int;
 
+	/**
+	 * Returns whether the given slot is empty.
+	 *
+	 * @param int $index
+	 *
+	 * @return bool
+	 */
+	public function isSlotEmpty(int $index) : bool;
+
 	/**
 	 * Will remove all the Items that has the same id and metadata (if not null)
 	 *