Fix crash in DebugBillingClient when billing returns error/empty result.

DebugBillingClient.kt

private val broadcastReceiver = object : BroadcastReceiver() {
    override fun onReceive(context: Context?, intent: Intent?) {
      // Receiving the result from local broadcast and triggering a callback on listener.
      @BillingResponse
      val responseCode = intent?.getIntExtra(DebugBillingActivity.RESPONSE_CODE, BillingResponse.ERROR)
          ?: BillingResponse.ERROR

      var purchases: List<Purchase>? = null
      if (responseCode == BillingResponse.OK) {
        val resultData = intent?.getBundleExtra(DebugBillingActivity.RESPONSE_BUNDLE)
        purchases = BillingHelper.extractPurchases(resultData)

        // save the purchase
        purchases.forEach { billingStore.addPurchase(it) }
      }

      // save the result
      purchasesUpdatedListener.onPurchasesUpdated(responseCode, purchases)
    }
  }