docs: @NonNull
      public static SourceParams createSofortParams(
              @IntRange(from = 0) long amount,
              @NonNull String returnUrl,
              @NonNull @Size(2) String country,
              @Nullable String statementDescriptor) {
          final SourceParams params = new SourceParams()
                  .setType(Source.SOFORT)
                  .setCurrency(Source.EURO)
                  .setAmount(amount)
                  .setRedirect(createSimpleMap(FIELD_RETURN_URL, returnUrl));
  
          final Map<String, Object> sofortMap = createSimpleMap(FIELD_COUNTRY, country);
          if (statementDescriptor != null) {
              sofortMap.put(FIELD_STATEMENT_DESCRIPTOR, statementDescriptor);
          }
  
          return params
                  .setApiParameterMap(sofortMap);
      }