Added check for charts without series data.
Added check for charts without series data.

Issue #326.

jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jkyeung/XlsxWriter,jmcnamara/XlsxWriter,jkyeung/XlsxWriter,jkyeung/XlsxWriter,jmcnamara/XlsxWriter,jkyeung/XlsxWriter