Fix error when cleaning up MR with no head ref

When `MergeRequests::CleanupRefsService` runs on a merge request
that has no head ref (`refs/merge-requests/:iid/head`), it throws
a `NoMethodError` when getting the commit id.

To fix it, there's ensure that we handle that case.

app/services/merge_requests/cleanup_refs_service.rb

@@ -17,7 +17,7 @@ module MergeRequests
       @repository = merge_request.project.repository
       @ref_path = merge_request.ref_path
       @merge_ref_path = merge_request.merge_ref_path
-      @ref_head_sha = @repository.commit(merge_request.ref_path).id
+      @ref_head_sha = @repository.commit(merge_request.ref_path)&.id
       @merge_ref_sha = merge_request.merge_ref_head&.id
     end
 

changelogs/unreleased/270054-fix-no-method-error.yml

+---
+title: Fix error when cleaning up MR with no head ref
+merge_request: 45504
+author:
+type: fixed

spec/services/merge_requests/cleanup_refs_service_spec.rb

@@ -35,6 +35,17 @@ RSpec.describe MergeRequests::CleanupRefsService do
         end
       end
 
+      context 'when merge request has no head ref' do
+        before do
+          # Simulate a merge request with no head ref
+          merge_request.project.repository.delete_refs(merge_request.ref_path)
+        end
+
+        it 'does not fail' do
+          expect(result[:status]).to eq(:success)
+        end
+      end
+
       context 'when merge request has merge ref' do
         before do
           MergeRequests::MergeToRefService