fix issue with splitting defined named

[rogsmith]

When trying to call doc.labels, I was receiving exception: NoMethodError: undefined method `split' for nil:NilClass because the defined_name.text was not in the format expected.

[stevendaniels]

Thanks for the PR.

This certainly prevents the error in #276 from being raised, but it doesn't fix the issue. I'd still like to understand what's going wrong in your case, write a test case for the situation and apply an appropriate fix.

If you look at the comments for that method, it's clear it's looking for a defined name similar to this value: "Sheet1!$C$5". My first question is what is your defined_name's value? Then, I'd want to figure out if your defined_name's value is a valid value for that field (some XLSX generators make mistakes). If it is a valid value for that field, then we'd start looking for a fix.