payments/migration1: handle legacy payments with nil HTLC hash

For legacy payments, the HTLC Hash field may be nil in the bbolt
backend. Previously, the migration would fail with "HTLC attempt X
missing payment hash" when encountering such payments.

This commit fixes the migration by falling back to the parent payment
hash when the HTLC-specific hash is nil. This is consistent with how
the router handles legacy payments (see patchLegacyPaymentHash in
payment_lifecycle.go).

The validation logic is also updated to apply the same fallback when
comparing bbolt data with migrated SQL data, ensuring the comparison
succeeds.

payments/db/migration1/migration_validation.go

@@ -299,6 +299,17 @@ func normalizePaymentForCompare(payment *MPPayment) {
 		htlc.circuit = nil
 		htlc.cachedSessionKey = nil
 
+		// For legacy payments, the HTLC Hash field may be nil in the
+		// bbolt backend. During migration, the SQL code uses the
+		// parent payment hash as fallback. To ensure the comparison
+		// between bbolt and SQL data succeeds, we apply the same
+		// fallback here.
+		//
+		// See also: patchLegacyPaymentHash in payment_lifecycle.go.
+		if htlc.Hash == nil && payment.Info != nil {
+			htlc.Hash = &payment.Info.PaymentIdentifier
+		}
+
 		if len(htlc.Route.FirstHopWireCustomRecords) == 0 {
 			htlc.Route.FirstHopWireCustomRecords =
 				lnwire.CustomRecords{}