Add guard condition for when cilium

is not installed but there NetworkPolicies.
Payload will be returned while the error
message related to CiliumNetworkPolicy is kept.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63258
EE: true

ee/app/services/network_policies/resources_service.rb

@@ -18,7 +18,7 @@ module NetworkPolicies
 
       @kubeclient_info.each do |platform, namespace|
         policies_per_environment, error_per_environment = execute_per_environment(platform, namespace)
-        policies += policies_per_environment
+        policies += policies_per_environment if policies_per_environment
         errors << error_per_environment if error_per_environment
       end
       errors.empty? ? ServiceResponse.success(payload: policies) : kubernetes_error_response(errors.join, policies)
@@ -35,7 +35,7 @@ module NetworkPolicies
         .map { |resource| Gitlab::Kubernetes::CiliumNetworkPolicy.from_resource(resource) }
       [policies, nil]
     rescue Kubeclient::HttpError => e
-      [[], e]
+      [policies, e]
     end
 
     def has_deployment_platform?(kubeclient_info)

ee/spec/services/network_policies/resources_service_spec.rb

@@ -56,7 +56,7 @@ RSpec.describe NetworkPolicies::ResourcesService do
       end
     end
 
-    context 'with Kubeclient::HttpError' do
+    context 'with Kubeclient::HttpError related to network policies' do
       before do
         allow(kubeclient).to receive(:get_network_policies).and_raise(Kubeclient::HttpError.new(500, 'system failure', nil))
       end
@@ -69,6 +69,20 @@ RSpec.describe NetworkPolicies::ResourcesService do
       end
     end
 
+    context 'with Kubeclient::HttpError related to cilium network policies' do
+      before do
+        allow(kubeclient).to receive(:get_network_policies) { [policy.generate] }
+        allow(kubeclient).to receive(:get_cilium_network_policies).and_raise(Kubeclient::HttpError.new(400, 'not found', nil))
+      end
+
+      it 'returns error response' do
+        expect(subject).to be_error
+        expect(subject.http_status).to eq(:bad_request)
+        expect(subject.message).not_to be_nil
+        expect(subject.payload.first.as_json).to eq(policy.as_json)
+      end
+    end
+
     context 'without environment_id' do
       let(:environment_id) { nil }
       let(:cluster_2) { create(:cluster, :project) }