Guard against nil file key

Not all scanners populate the file key in the location hash, so we need
to guard against that.

ee/app/presenters/vulnerabilities/occurrence_presenter.rb

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

ee/spec/presenters/vulnerabilities/occurrence_presenter_spec.rb

@@ -46,6 +46,15 @@ describe Vulnerabilities::OccurrencePresenter do
         it { is_expected.to end_with('#L1-2') }
       end
 
+      context 'without file' do
+        before do
+          allow(presenter).to receive(:location)
+            .and_return({ 'foo' => 123 })
+        end
+
+        it { is_expected.to be_blank }
+      end
+
       context 'without location' do
         before do
           allow(presenter).to receive(:location)