Fix Error 500 when viewing a merge request with a deleted source project

If the merge request can be rebased based on the history of the commits,
users attempting to view the merge request would hit an Error 500. This
occurred because `user_can_push_to_source_branch?` would be called
on a non-existent project.

Closes gitlab-org/gitlab-ce#33011

app/presenters/merge_request_presenter.rb

@@ -182,6 +182,8 @@ class MergeRequestPresenter < Gitlab::View::Presenter::Delegated
   end
 
   def user_can_push_to_source_branch?
+    return false unless source_branch_exists?
+
     ::Gitlab::UserAccess
       .new(current_user, project: source_project)
       .can_push_to_branch?(source_branch)

spec/serializers/merge_request_entity_spec.rb

@@ -146,4 +146,20 @@ describe MergeRequestEntity do
       end
     end
   end
+
+  describe 'when source project is deleted' do
+    let(:project) { create(:project, :repository) }
+    let(:fork_project) { create(:project, :repository, forked_from_project: project) }
+    let(:merge_request) { create(:merge_request, source_project: fork_project, target_project: project) }
+
+    it 'returns a blank rebase_path' do
+      allow(merge_request).to receive(:should_be_rebased?).and_return(true)
+      fork_project.destroy
+      merge_request.reload
+
+      entity = described_class.new(merge_request, request: request).as_json
+
+      expect(entity[:rebase_path]).to be_nil
+    end
+  end
 end