Fix MergeRequestService erroring out on deleted branch

When a branch is deleted, the push commit IDs is an empty array.
The previous change would attempt to call `[].exists?`, which is
invalid. Fix this by returning `MergeRequestDiffCommit.none` instead.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53853

app/models/merge_request_diff.rb

@@ -142,7 +142,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def commits_by_shas(shas)
-    return [] unless shas.present?
+    return MergeRequestDiffCommit.none unless shas.present?
 
     merge_request_diff_commits.where(sha: shas)
   end

spec/models/merge_request_spec.rb

@@ -2650,6 +2650,10 @@ describe MergeRequest do
   end
 
   describe '#includes_any_commits?' do
+    it 'returns false' do
+      expect(subject.includes_any_commits?([])).to be_falsey
+    end
+
     it 'returns false' do
       expect(subject.includes_any_commits?([Gitlab::Git::BLANK_SHA])).to be_falsey
     end