Fixing https://github.com/pyexcel/pyexcel-ods3/issues/36

[mleiseca]

Here's the location in the stack trace:

File "pyexcel_ods3/odsr.py", line 46, in cell_value
    ret = str(cell_value) + " " + cell.currency
          ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
TypeError: can only concatenate str (not "NoneType") to str

With your PR, here is a check list:

• Has test cases written?
• Has all code lines tested?
• [?] Has make format been run? -> This yields changes unrelated to my PR. Do you want me to add these?
• Please update CHANGELOG.yml(not CHANGELOG.rst)
• Has fair amount of documentation if your change is complex - Not needed, this is just a bug fix
• Agree on NEW BSD License for your contribution