Fixed error with useBreakOn parameters

A E_RECOVERABLE_ERROR error happened: "Argument 1 passed to pocketmine\block\Block::getDrops() must be an instance of pocketmine\item\Item, null given

AND

A E_RECOVERABLE_ERROR error happened: "Argument 1 passed to pocketmine\block\Generic::onBreak() must be an instance of pocketmine\item\Item, null given

are fixed
Merge pull request #1772 from Striker209MPE/patch-3

Fixed error with useBreakOn parameters