Fix NO unbond BP

This MR fixes the issue when a Node Operator want to unbond a Bond Provider with a UNBOND:NodeAddress:BPAddress memo. Because the memo doesn't have an specific ASSET, this parameter is empty and when the function calcLiquidityInCacao() is called it gets an empty value.

Removed test "fail to refund rewards should return an error" in x/mayachain/handler_leave_test.go because it doesn't apply to maya since it test the function call TryAddTxOutItem() which is not present on refundBond().