exer2k

Fix ValueError when date type cell is empty

1

Resolves #3.

TypeError is raised when cell with date type is empty

1

> - **XML2XLSX version:** 1.0.1 > - **Python interpreter version:** 3.7.3 > - **Platform:** Darwin-18.6.0-x86_64-i386-64bit ------------ Assume that there is a cell with `type` attribute set to `date`, which has...