message: Make 'source' field in PaymentIntent struct optional
  Found out that source field can be missing in payment intent
diff: @@ -321,7 +321,7 @@ pub struct PaymentIntent {
  pub receipt_email: Option<String>,
  pub review: Option<String>,
  pub shipping: Option<ShippingDetails>,
  - pub source: String,
  + pub source: Option<String>,
  pub statement_descriptor: Option<String>,
  pub status: PaymentIntentStatus,
  pub transfer_data: Option<TransferData>,
  