Fix issue CSV export failing for some projects

Some imported projects have NULL values for `label_links.label_id` for
some reason. This caused the issue export CSV to fail because the
exporter attempted to sort an array containing string values and `nil`
values.

We should clean up the database entries
(https://gitlab.com/gitlab-org/gitlab/issues/197307) for this with a NOT
NULL constraint, but this may require some careful consideration. For
now, to make exports work again, we just reject any invalid label links.

Closes https://gitlab.com/gitlab-org/gitlab/issues/197300

changelogs/unreleased/sh-fix-issue-197300.yml

+---
+title: Fix issue CSV export failing for some projects
+merge_request: 23223
+author:
+type: fixed

ee/app/models/concerns/ee/issuable.rb

@@ -11,7 +11,7 @@ module EE
 
         relation = unscoped.where(id: self.select(:id)).eager_load(:labels)
         relation.pluck(:id, 'labels.title').each do |issue_id, label|
-          issue_labels[issue_id] << label
+          issue_labels[issue_id] << label if label.present?
         end
 
         issue_labels

ee/spec/models/concerns/ee/issuable_spec.rb

@@ -37,6 +37,23 @@ describe EE::Issuable do
 
       expect(relation.labels_hash[issue_id]).to include('Feature', 'Second Label')
     end
+
+    # This tests the workaround for the lack of a NOT NULL constraint in
+    # label_links.label_id:
+    # https://gitlab.com/gitlab-org/gitlab/issues/197307
+    context 'with a NULL label ID in the link' do
+      let(:issue) { create(:labeled_issue, labels: [feature_label, second_label]) }
+
+      before do
+        label_link = issue.label_links.find_by(label_id: second_label.id)
+        label_link.label_id = nil
+        label_link.save(validate: false)
+      end
+
+      it 'filters out bad labels' do
+        expect(Issue.where(id: issue.id).labels_hash[issue.id]).to match_array(['Feature'])
+      end
+    end
   end
 
   describe '#matches_cross_reference_regex?' do