Fix token holders' tab at the Token's page

This page was breaking when the given Token has the total supply nil.

To fix that, we added a view function to check if we should show the
total supply percentage. Since we want to display just for Tokens that
have total supply, we are ignoring for the ones that don't have.

apps/block_scout_web/lib/block_scout_web/templates/tokens/holder/_token_balances.html.eex

@@ -10,7 +10,7 @@
           <%= format_token_balance_value(@token_balance.value, @token) %> <%= @token.symbol %>
         </span>
 
-        <%= if @token.total_supply > 0 do %>
+        <%= if show_total_supply_percentage?(@token.total_supply) do %>
           (<%= total_supply_percentage(@token_balance.value, @token.total_supply) %>)
         <% end %>
       </span>

apps/block_scout_web/lib/block_scout_web/views/tokens/holder_view.ex

@@ -4,6 +4,24 @@ defmodule BlockScoutWeb.Tokens.HolderView do
   alias BlockScoutWeb.Tokens.{OverviewView, TokenView}
   alias Explorer.Chain.{Token}
 
+  @doc """
+  Checks if the total supply percentage must be shown.
+
+  ## Examples
+
+    iex> BlockScoutWeb.Tokens.HolderView.show_total_supply_percentage?(nil)
+    false
+
+    iex> BlockScoutWeb.Tokens.HolderView.show_total_supply_percentage?(0)
+    false
+
+    iex> BlockScoutWeb.Tokens.HolderView.show_total_supply_percentage?(100)
+    true
+
+  """
+  def show_total_supply_percentage?(nil), do: false
+  def show_total_supply_percentage?(total_supply), do: total_supply > 0
+
   @doc """
   Calculates the percentage of the value from the given total supply.
 

apps/block_scout_web/test/block_scout_web/views/tokens/holder_view_test.exs

@@ -6,6 +6,26 @@ defmodule BlockScoutWeb.Tokens.HolderViewTest do
 
   doctest BlockScoutWeb.Tokens.HolderView, import: true
 
+  describe "show_total_supply_percentage?/1" do
+    test "returns false when the total supply is nil" do
+      %Token{total_supply: total_supply} = build(:token, total_supply: nil)
+
+      refute HolderView.show_total_supply_percentage?(total_supply)
+    end
+
+    test "returns false when the total supply is 0" do
+      %Token{total_supply: total_supply} = build(:token, total_supply: 0)
+
+      refute HolderView.show_total_supply_percentage?(total_supply)
+    end
+
+    test "returns true when the total supply is greater than 0" do
+      %Token{total_supply: total_supply} = build(:token, total_supply: 1000)
+
+      assert HolderView.show_total_supply_percentage?(total_supply)
+    end
+  end
+
   describe "total_supply_percentage/2" do
     test "returns the percentage of the Token total supply" do
       %Token{total_supply: total_supply} = build(:token, total_supply: 1000)