FIX: Handle nil values in DistributedCache#defer_get_set (#15978)

Themes often cache `nil` values in a DistributedCache. This bug meant that we were re-calculating some values on every request, AND triggering message-bus publishing on every request.

This fix should provide a significant performance improvement for busy sites.

spec/lib/distributed_cache_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe "DistributedCache extensions" do
+  let(:cache) { DistributedCache.new('mytest') }
+
+  it "can defer_get_set" do
+    messages = MessageBus.track_publish("/distributed_hash") do
+      cache.defer_get_set("key") { "value" }
+    end
+    expect(messages.size).to eq(1)
+    expect(cache["key"]).to eq("value")
+  end
+
+  it "works correctly for nil values" do
+    block_called_counter = 0
+    messages = MessageBus.track_publish("/distributed_hash") do
+      2.times do
+        cache.defer_get_set("key") do
+          block_called_counter += 1
+          nil
+        end
+      end
+    end
+
+    expect(block_called_counter).to eq(1)
+    expect(messages.size).to eq(1)
+    expect(cache["key"]).to eq(nil)
+  end
+end