Error when opening file with no shared strings

[degaart]

If a spreadsheet has no shared strings (it's rare, but excel can produce such files), an exception is thrown in shared_strings_serializer::read_shared_strings() Specifically, it's on the line:

auto unique_count = std::stoull(root_node.get_attribute("uniqueCount"));

The solution would be to check if root_node has an "uniqueCount" attribute, else we consider unique_count to be zero.

I can produce a pull request if you want

[tfussell]

Interesting. I should have had a check for that. Thanks for reporting the issue. I'll try to get it sorted soon.

[tfussell]

This should be fixed now. Let me know if you have any more problems.