Add guard - diff missing from merge_request_diffs

If merge_request_diff for some reason isn't contained in
@merge_request_diffs, we will see a TypeError

Changelog: fixed

app/serializers/merge_request_diff_entity.rb

@@ -9,7 +9,7 @@ class MergeRequestDiffEntity < Grape::Entity
     @merge_request_diffs = options[:merge_request_diffs]
     diff = options[:merge_request_diff]
 
-    next unless diff.present?
+    next unless @merge_request_diffs.include?(diff)
     next unless @merge_request_diffs.size > 1
 
     version_index(merge_request_diff)

spec/serializers/merge_request_diff_entity_spec.rb

@@ -11,7 +11,13 @@ RSpec.describe MergeRequestDiffEntity do
   let(:merge_request_diff) { merge_request_diffs.first }
 
   let(:entity) do
-    described_class.new(merge_request_diff, 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_diff: merge_request_diff,
+      merge_request_diffs: merge_request_diffs
+    )
   end
 
   subject { entity.as_json }
@@ -26,6 +32,46 @@ RSpec.describe MergeRequestDiffEntity do
     end
   end
 
+  describe '#version_index' do
+    shared_examples 'version_index is nil' do
+      it 'returns nil' do
+        expect(subject[:version_index]).to be_nil
+      end
+    end
+
+    context 'when diff is not present' do
+      let(:entity) do
+        described_class.new(
+          merge_request_diff,
+          request: request,
+          merge_request: merge_request,
+          merge_request_diffs: merge_request_diffs
+        )
+      end
+
+      it_behaves_like 'version_index is nil'
+    end
+
+    context 'when diff is not included in @merge_request_diffs' do
+      let(:merge_request_diff) { create(:merge_request_diff) }
+      let(:merge_request_diff_2) { create(:merge_request_diff) }
+
+      before do
+        merge_request_diffs << merge_request_diff_2
+      end
+
+      it_behaves_like 'version_index is nil'
+    end
+
+    context 'when @merge_request_diffs.size <= 1' do
+      before do
+        expect(merge_request_diffs.size).to eq(1)
+      end
+
+      it_behaves_like 'version_index is nil'
+    end
+  end
+
   describe '#short_commit_sha' do
     it 'returns short sha' do
       expect(subject[:short_commit_sha]).to eq('b83d6e39')