Fix handling of deleted values in several places. (#241)

Now that the m_entries entry is kept, other places can't assume that
a value exists simply because the entry exists.

src/main/native/cpp/Storage.h

@@ -251,7 +251,7 @@ class Storage : public IStorage {
   template <typename F>
   void DeleteAllEntriesImpl(bool local, F should_delete);
   void DeleteAllEntriesImpl(bool local);
-  Entry* GetOrNew(StringRef name, bool* is_new = nullptr);
+  Entry* GetOrNew(StringRef name);
 };
 
 }  // namespace nt