Prevent placement of unknown blocks, closes #2260

I don't know why I ever allowed this in the first place... stupid idea...

src/pocketmine/block/UnknownBlock.php

@@ -31,6 +31,10 @@ class UnknownBlock extends Transparent{
 		return 0;
 	}
 
+	public function canBePlaced() : bool{
+		return false;
+	}
+
 	public function getDrops(Item $item) : array{
 		return [];
 	}