Protect against empty location attribute

Not all scanners populate the location attribute, which causes an error
that will prevent the security dashboard from loading.

ee/app/presenters/vulnerabilities/occurrence_presenter.rb

@@ -6,6 +6,7 @@ module Vulnerabilities
 
     def blob_path
       return '' unless respond_to?(:sha)
+      return '' unless location.present?
 
       add_line_numbers(location['start_line'], location['end_line'])
     end

ee/spec/presenters/vulnerabilities/occurrence_presenter_spec.rb

@@ -45,6 +45,15 @@ describe Vulnerabilities::OccurrencePresenter do
 
         it { is_expected.to end_with('#L1-2') }
       end
+
+      context 'without location' do
+        before do
+          allow(presenter).to receive(:location)
+            .and_return({})
+        end
+
+        it { is_expected.to be_blank }
+      end
     end
   end
 end