Fix displaying blocking MRs with no metrics

Metrics may not exist for some older MRs, so we need to &.

ee/app/serializers/blocking_merge_request_entity.rb

@@ -29,7 +29,7 @@ class BlockingMergeRequestEntity < Grape::Entity
   expose :created_at
   expose :merged_at
   expose :closed_at do |blocking_mr|
-    blocking_mr.metrics.latest_closed_at
+    blocking_mr.metrics&.latest_closed_at
   end
 
   private

ee/changelogs/unreleased/209853-fix-blocking-mr-latest-closed-at.yml

+---
+title: Fix displaying merge request dependencies with no metrics
+merge_request: 49466
+author:
+type: fixed

ee/spec/serializers/blocking_merge_request_entity_spec.rb

@@ -29,6 +29,12 @@ RSpec.describe BlockingMergeRequestEntity do
     )
   end
 
+  it 'serializes a blocking MR that lacks metrics' do
+    expect(merge_request).to receive(:metrics).and_return(nil)
+
+    expect(entity.as_json).to include(id: merge_request.id, closed_at: nil)
+  end
+
   describe '#head_pipeline' do
     subject { entity.as_json[:head_pipeline] }