Do not have a cache version if there's no version present

We should not cache empty responses.

app/models/alchemy/page/page_natures.rb

@@ -104,7 +104,7 @@ def layout_partial_name
       # Returns the string version of the +last_modified_at+ timestamp.
       #
       def cache_version
-        last_modified_at.to_s
+        last_modified_at&.to_s
       end
 
       # Returns the timestamp that the page was last modified at, regardless of through.
@@ -117,7 +117,7 @@ def cache_version
       #
       def last_modified_at
         relevant_page_version = (Current.preview_page == self) ? draft_version : public_version
-        relevant_page_version&.updated_at || updated_at
+        relevant_page_version&.updated_at
       end
 
       # Returns true if the page cache control headers should be set.

spec/models/alchemy/page_spec.rb

@@ -706,18 +706,30 @@ module Alchemy
     describe "#cache_version" do
       let(:page) { build(:alchemy_page) }
 
-      before do
-        allow(page).to receive(:last_modified_at).and_return(1.day.ago)
-      end
-
       around do |example|
         travel_to(Time.parse("2019-01-01 12:00:00 UTC")) do
           example.run
         end
       end
 
-      it "returns a cache version string" do
-        expect(page.cache_version).to eq("2018-12-31 12:00:00 UTC")
+      context "last modified is a time object" do
+        before do
+          allow(page).to receive(:last_modified_at).and_return(1.day.ago)
+        end
+
+        it "returns a cache version string" do
+          expect(page.cache_version).to eq("2018-12-31 12:00:00 UTC")
+        end
+      end
+
+      context "last modified at is nil" do
+        before do
+          allow(page).to receive(:last_modified_at).and_return(nil)
+        end
+
+        it "returns a cache version string" do
+          expect(page.cache_version).to be(nil)
+        end
       end
     end
 
@@ -768,17 +780,17 @@ module Alchemy
           context "if page has no draft version" do
             let(:draft_version) { nil }
 
-            it "uses page's updated_at" do
-              is_expected.to be_within(1.second).of(yesterday)
+            it "is nil" do
+              is_expected.to be(nil)
             end
           end
         end
 
         context "not in preview mode" do
           let(:preview) { nil }
 
-          it "uses page's updated_at" do
-            is_expected.to be_within(1.second).of(yesterday)
+          it "is nil" do
+            is_expected.to be(nil)
           end
         end
       end