use nil as a default value for transaction cache

apps/explorer/lib/explorer/chain.ex

@@ -1871,7 +1871,7 @@ defmodule Explorer.Chain do
   def transaction_estimated_count do
     cached_value = TransactionCountCache.value()
 
-    if cached_value == 0 do
+    if is_nil(cached_value) do
       %Postgrex.Result{rows: [[rows]]} =
         SQL.query!(Repo, "SELECT reltuples::BIGINT AS estimate FROM pg_class WHERE relname='transactions'")
 

apps/explorer/lib/explorer/chain/transaction_count_cache.ex

@@ -12,7 +12,7 @@ defmodule Explorer.Chain.TransactionCountCache do
 
   # 2 hours
   @cache_period 1_000 * 60 * 60 * 2
-  @default_value 0
+  @default_value nil
   @key "count"
   @name __MODULE__
 

apps/explorer/test/explorer/chain/transaction_count_cache_test.exs

@@ -8,7 +8,7 @@ defmodule Explorer.Chain.TransactionCountCacheTest do
 
     result = TransactionCountCache.value(TestCache)
 
-    assert result == 0
+    assert is_nil(result)
   end
 
   test "updates cache if initial value is zero" do