Fix using link to uploads in global snippets

Closes #17342, closes #17363

lib/banzai/filter/upload_link_filter.rb

@@ -31,7 +31,9 @@ def process_link_attr(html_attr)
       end
 
       def build_url(uri)
-        File.join(Gitlab.config.gitlab.url, context[:project].path_with_namespace, uri)
+        return '#' unless project = context[:project]
+
+        File.join(Gitlab.config.gitlab.url, project.path_with_namespace, uri)
       end
 
       # Ensure that a :project key exists in context

spec/lib/banzai/filter/upload_link_filter_spec.rb

@@ -8,6 +8,10 @@ def filter(doc, contexts = {})
       project: project
     })
 
+    raw_filter(doc, contexts)
+  end
+
+  def raw_filter(doc, contexts = {})
     described_class.call(doc, contexts)
   end
 
@@ -70,4 +74,18 @@ def link(path)
       expect(doc.at_css('img')['src']).to match "#{Gitlab.config.gitlab.url}/#{project.path_with_namespace}/uploads/%ED%95%9C%EA%B8%80.png"
     end
   end
+
+  context 'when project context does not exist' do
+    let(:upload_link) { link('/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg') }
+
+    it 'does not raise error' do
+      expect { raw_filter(upload_link, project: nil) }.to_not raise_error
+    end
+
+    it 'provides an empty link' do
+      doc = raw_filter(upload_link, project: nil)
+
+      expect(doc.at_css('a')['href']).to eq '#'
+    end
+  end
 end