fix tests

apps/block_scout_web/lib/block_scout_web/views/chain_view.ex

@@ -3,8 +3,13 @@ defmodule BlockScoutWeb.ChainView do
 
   alias BlockScoutWeb.LayoutView
 
-  defp market_cap(:standard, exchange_rate) do
-    Decimal.mult(exchange_rate.available_supply, exchange_rate.usd_value)
+  defp market_cap(:standard, %{available_supply: available_supply, usd_value: usd_value})
+       when is_nil(available_supply) or is_nil(usd_value) do
+    Decimal.new(0)
+  end
+
+  defp market_cap(:standard, %{available_supply: available_supply, usd_value: usd_value}) do
+    Decimal.mult(available_supply, usd_value)
   end
 
   defp market_cap(module, exchange_rate) do