Don't attempt an upload when all cookbooks exist on the server

Chef Server treats an API request to upload zero files as an error, so
skip the upload when all required cookbooks are present on the server.

lib/chef-dk/policyfile/uploader.rb

@@ -119,7 +119,7 @@ def cookbook_versions_for_policy
       def upload_cookbooks
         ui.msg("WARN: Uploading cookbooks using semver compat mode")
 
-        uploader.upload_cookbooks
+        uploader.upload_cookbooks unless cookbook_versions_to_upload.empty?
 
         reused_cbs, uploaded_cbs = cookbook_versions_for_policy.partition do |cb_with_lock|
           remote_already_has_cookbook?(cb_with_lock.cookbook)

spec/unit/policyfile/uploader_spec.rb

@@ -283,6 +283,37 @@ def lock_double(name, dotted_decimal_id)
 
     end
 
+    context "with a set of cookbooks that all exist on the server" do
+
+      before do
+        # Have this one:
+        lock_double("build-essential", "67369247788170534.26353953100055918.55660493423796")
+      end
+
+      let(:expected_cookbooks_for_upload) do
+        []
+      end
+
+      it "lists no cookbooks as needing to be uploaded" do
+        expect(policyfile_lock).to receive(:validate_cookbooks!)
+        expect(http_client).to receive(:get).with('cookbooks?num_versions=all').and_return(existing_cookbook_on_remote)
+
+        expect(uploader.cookbook_versions_to_upload).to eq(expected_cookbooks_for_upload)
+      end
+
+      it "skips cookbooks uploads, then uploads the policy" do
+        expect(policyfile_lock).to receive(:validate_cookbooks!)
+        expect(http_client).to receive(:get).with('cookbooks?num_versions=all').and_return(existing_cookbook_on_remote)
+
+        expect(uploader.uploader).to_not receive(:upload_cookbooks)
+
+        # behavior for these tested above
+        expect(uploader).to receive(:data_bag_create)
+        expect(uploader).to receive(:data_bag_item_create)
+
+        uploader.upload
+      end
+    end
   end
 
 end