Match on nil so that Ecto NotLoaded raises a no function match error

If an Address association isn’t preloaded, we want the page to crash for
now since it’s not valid to try to render an address that hasn’t been 
fetched.

Co-authored-by: Stamates <stamates@hotmail.com>

apps/explorer_web/lib/explorer_web/views/address_view.ex

@@ -35,7 +35,7 @@ defmodule ExplorerWeb.AddressView do
 
   def contract?(%Address{contract_code: _}), do: true
 
-  def contract?(_), do: true
+  def contract?(nil), do: true
 
   def formatted_usd(%Address{fetched_balance: nil}, _), do: nil