Serialize short sha as nil if head commit is blank

app/serializers/merge_request_diff_entity.rb

@@ -21,6 +21,8 @@ class MergeRequestDiffEntity < Grape::Entity
   expose :latest?, as: :latest
 
   expose :short_commit_sha do |merge_request_diff|
+    next unless merge_request_diff.head_commit_sha
+
     short_sha(merge_request_diff.head_commit_sha)
   end
 

changelogs/unreleased/id-nil-short-commit-sha.yml

+---
+title: Serialize short sha as nil if head commit is blank
+merge_request: 19014
+author:
+type: fixed

spec/serializers/merge_request_diff_entity_spec.rb

@@ -7,14 +7,15 @@ describe MergeRequestDiffEntity do
   let(:request) { EntityRequest.new(project: project) }
   let(:merge_request) { create(:merge_request_with_diffs, target_project: project, source_project: project) }
   let(:merge_request_diffs) { merge_request.merge_request_diffs }
+  let(:merge_request_diff) { merge_request_diffs.first }
 
   let(:entity) do
-    described_class.new(merge_request_diffs.first, request: request, merge_request: merge_request, merge_request_diffs: merge_request_diffs)
+    described_class.new(merge_request_diff, request: request, merge_request: merge_request, merge_request_diffs: merge_request_diffs)
   end
 
-  context 'as json' do
   subject { entity.as_json }
 
+  context 'as json' do
     it 'exposes needed attributes' do
       expect(subject).to include(
         :version_index, :created_at, :commits_count,
@@ -23,4 +24,16 @@ describe MergeRequestDiffEntity do
       )
     end
   end
+
+  describe '#short_commit_sha' do
+    it 'returns short sha' do
+      expect(subject[:short_commit_sha]).to eq('b83d6e39')
+    end
+
+    it 'returns nil if head_commit_sha does not exist' do
+      allow(merge_request_diff).to receive(:head_commit_sha).and_return(nil)
+
+      expect(subject[:short_commit_sha]).to eq(nil)
+    end
+  end
 end