fix: Update overview.html.eex (#11094)

apps/block_scout_web/lib/block_scout_web/templates/block/overview.html.eex

@@ -161,6 +161,7 @@
             <dd class="col-sm-9 col-lg-10"><%= @block.difficulty |> Decimal.to_integer() |> BlockScoutWeb.Cldr.Number.to_string! %></dd>
           </dl>
           <%= if block_type(@block) == "Block" do %>
+            <%= if !is_nil(@block.total_difficulty) do %>
               <!-- Total Difficulty -->
               <dl class="row">
                 <dt class="col-sm-3 col-lg-2 text-muted">
@@ -170,6 +171,7 @@
                 </dt>
                 <dd class="col-sm-9 col-lg-10"><%= @block.total_difficulty |> Decimal.to_integer() |> BlockScoutWeb.Cldr.Number.to_string! %></dd>
               </dl>
+            <% end %>
             <!-- Gas Used -->
             <dl class="row">
               <dt class="col-sm-3 col-lg-2 text-muted">