Fix NoMethodError for UnfoldPresenter

Sentry error:
https://sentry.gitlab.net/gitlab/gitlabcom/issues/3168538

Add a fallback when array fetch returns `nil`

Changelog: fixed

app/presenters/blobs/unfold_presenter.rb

@@ -108,7 +108,7 @@ module Blobs
     def limit(lines)
       return lines if full?
 
-      lines[since - 1..to - 1]
+      lines[since - 1..to - 1] || []
     end
   end
 end

spec/presenters/blobs/unfold_presenter_spec.rb

@@ -206,6 +206,14 @@ RSpec.describe Blobs::UnfoldPresenter do
       end
     end
 
+    context 'when since exceeds number of lines' do
+      let(:params) { { since: 2 } }
+
+      it 'returns an empty list' do
+        expect(subject.lines.size).to eq(0)
+      end
+    end
+
     context 'when full is true' do
       let(:params) { { full: true } }