Fix cross-project label ref with invalid project

Closes #15168

lib/banzai/filter/label_reference_filter.rb

@@ -20,7 +20,7 @@ module Banzai
         text.gsub(pattern) do |match|
           project = project_from_ref($~[:project])
           params = label_params($~[:label_id].to_i, $~[:label_name])
-          label = project.labels.find_by(params)
+          label = project.labels.find_by(params) if project
 
           if label
             yield match, label.id, $~[:project], $~

spec/lib/banzai/filter/label_reference_filter_spec.rb

@@ -178,6 +178,7 @@ describe Banzai::Filter::LabelReferenceFilter, lib: true do
   end
 
   describe 'cross project label references' do
+    context 'valid project referenced' do
       let(:another_project)  { create(:empty_project, :public) }
       let(:project_name) { another_project.name_with_namespace }
       let(:label) { create(:label, project: another_project, color: '#00ff00') }
@@ -201,4 +202,13 @@ describe Banzai::Filter::LabelReferenceFilter, lib: true do
         expect(result.css('a').first.text).to eq "#{label.name} in #{project_name}"
       end
     end
+
+    context 'project that does not exist referenced' do
+      let(:result) { reference_filter('aaa/bbb~ccc') }
+
+      it 'does not link reference' do
+        expect(result.to_html).to eq 'aaa/bbb~ccc'
+      end
+    end
+  end
 end