Add RedactorFilter specs for invalid Group and Project references

spec/lib/gitlab/markdown/redactor_filter_spec.rb

@@ -37,6 +37,12 @@ module Gitlab::Markdown
 
         expect(doc.css('a').length).to eq 1
       end
+
+      it 'handles invalid Group references' do
+        link = reference_link(group_id: 12345)
+
+        expect { filter(link) }.not_to raise_error
+      end
     end
 
     context 'with data-project-id' do
@@ -60,6 +66,12 @@ module Gitlab::Markdown
 
         expect(doc.css('a').length).to eq 1
       end
+
+      it 'handles invalid Project references' do
+        link = reference_link(project_id: 12345)
+
+        expect { filter(link) }.not_to raise_error
+      end
     end
 
     context 'with data-user-id' do