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

@@ -178,27 +178,37 @@ describe Banzai::Filter::LabelReferenceFilter, lib: true do
   end
 
   describe 'cross project label references' 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') }
-    let(:reference) { label.to_reference(project) }
+    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') }
+      let(:reference) { label.to_reference(project) }
 
-    let!(:result) { reference_filter("See #{reference}") }
+      let!(:result) { reference_filter("See #{reference}") }
 
-    it 'points to referenced project issues page' do
-      expect(result.css('a').first.attr('href'))
-        .to eq urls.namespace_project_issues_url(another_project.namespace,
-                                                 another_project,
-                                                 label_name: label.name)
-    end
+      it 'points to referenced project issues page' do
+        expect(result.css('a').first.attr('href'))
+          .to eq urls.namespace_project_issues_url(another_project.namespace,
+                                                   another_project,
+                                                   label_name: label.name)
+      end
+
+      it 'has valid color' do
+        expect(result.css('a span').first.attr('style'))
+          .to match /background-color: #00ff00/
+      end
 
-    it 'has valid color' do
-      expect(result.css('a span').first.attr('style'))
-        .to match /background-color: #00ff00/
+      it 'contains cross project content' do
+        expect(result.css('a').first.text).to eq "#{label.name} in #{project_name}"
+      end
     end
 
-    it 'contains cross project content' do
-      expect(result.css('a').first.text).to eq "#{label.name} in #{project_name}"
+    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