Handle raw_repository returning nil in exists?

If path_with_namespace is nil Repository#raw_repository will also return
nil. Apparently code out there creates a Repository instance without a
namespace path. Right.

app/models/repository.rb

@@ -38,6 +38,8 @@ class Repository
   end
 
   def exists?
+    return false unless raw_repository
+
     raw_repository.rugged
     true
   rescue Gitlab::Git::Repository::NoRepository

spec/models/repository_spec.rb

@@ -240,6 +240,12 @@ describe Repository, models: true do
 
       expect(repository.exists?).to eq(false)
     end
+
+    it 'returns false when there is no namespace' do
+      allow(repository).to receive(:path_with_namespace).and_return(nil)
+
+      expect(repository.exists?).to eq(false)
+    end
   end
 
   describe '#has_visible_content?' do