Don't crash when listing helm chart images

- Adds fixtures for helm charts
- Fix crash when trying to parse nil date due to helm chart
  missing the field

changelogs/unreleased/37972-container-registry-tags-expect-oci-image-configs-to-have-created-fi.yml

+---
+title: Fix crash registry contains helm charts
+merge_request: 21381
+author:
+type: fixed

lib/container_registry/tag.rb

@@ -82,7 +82,7 @@ module ContainerRegistry
       return unless config
 
       strong_memoize(:created_at) do
-        DateTime.rfc3339(config['created'])
+        DateTime.rfc3339(config['created']) if config['created']
       end
     end
 

spec/fixtures/container_registry/config_blob_helm.json

+{
+  "name": "mychart",
+  "version": "0.1.0",
+  "description": "A Helm chart for Kubernetes",
+  "apiVersion": "v2",
+  "appVersion": "1.16.0",
+  "type": "application"
+}

spec/fixtures/container_registry/tag_manifest_helm.json

+{
+  "schemaVersion": 2,
+  "config": {
+    "mediaType": "application/vnd.cncf.helm.config.v1+json",
+    "digest": "sha256:65a07b841ece031e6d0ec5eb948eacb17aa6d7294cdeb01d5348e86242951487",
+    "size": 141
+  },
+  "layers": [
+    {
+      "mediaType": "application/tar+gzip",
+      "digest": "sha256:896935a875c8fe8f8b9b81e5862413de316f8da3d6d9a7e0f6f1e90f6204f551",
+      "size": 2801
+    }
+  ]
+}

spec/lib/container_registry/tag_spec.rb

@@ -97,6 +97,29 @@ describe ContainerRegistry::Tag do
       end
     end
 
+    context 'image is a helm chart' do
+      before do
+        stub_request(:get, 'http://registry.gitlab/v2/group/test/manifests/tag')
+          .with(headers: headers)
+          .to_return(
+            status: 200,
+            body: File.read(Rails.root + 'spec/fixtures/container_registry/tag_manifest_helm.json'),
+            headers: { 'Content-Type' => 'application/vnd.docker.distribution.manifest.v2+json' })
+
+        stub_request(:get, 'http://registry.gitlab/v2/group/test/blobs/sha256:65a07b841ece031e6d0ec5eb948eacb17aa6d7294cdeb01d5348e86242951487')
+          .with(headers: { 'Accept' => 'application/vnd.cncf.helm.config.v1+json' })
+          .to_return(
+            status: 200,
+            body: File.read(Rails.root + 'spec/fixtures/container_registry/config_blob_helm.json'))
+      end
+
+      context '#created_at' do
+        subject { tag.created_at }
+
+        it { is_expected.to be_nil }
+      end
+    end
+
     context 'schema v2' do
       before do
         stub_request(:get, 'http://registry.gitlab/v2/group/test/manifests/tag')