fix(ods): None values are not displayed as string

[hoccau]

Actually, None values are converted in string in an ODS sheet (so 'None' string is displayed in cells). This fill a cell empty value in case of None.
Sorry for not providing test : I see that ODS is tested only globally. It could be improved later :)
Thanks!

[codecov]

Codecov Report

Merging #566 (aec46cb) into master (8e2004c) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #566   +/-   ##
=======================================
  Coverage   92.40%   92.41%           
=======================================
  Files          28       28           
  Lines        2938     2941    +3     
=======================================
+ Hits         2715     2718    +3     
  Misses        223      223           

Files	Coverage Δ
src/tablib/_vendor/dbfpy/dbf.py	68.75% <ø> (ø)
src/tablib/_vendor/dbfpy/dbfnew.py	97.67% <ø> (ø)
src/tablib/_vendor/dbfpy/fields.py	71.27% <ø> (ø)
src/tablib/_vendor/dbfpy/header.py	83.16% <ø> (ø)
src/tablib/_vendor/dbfpy/record.py	76.38% <ø> (ø)
src/tablib/_vendor/dbfpy/utils.py	90.32% <ø> (ø)
src/tablib/formats/_dbf.py	100.00% <100.00%> (ø)
src/tablib/formats/_ods.py	99.24% <100.00%> (+0.01%)	⬆️
tests/test_tablib.py	98.84% <100.00%> (+<0.01%)	⬆️
tests/test_tablib_dbfpy_packages_fields.py	100.00% <100.00%> (ø)
... and 1 more

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[claudep]

I would suggest waiting for PR #568 to be merged, then you can rebase and take advantage of the new import functionality to complement the tests.

[hoccau]

I would suggest waiting for PR #568 to be merged, then you can rebase and take advantage of the new import functionality to complement the tests.

ok, fine.

[claudep]

Thanks! That looks good.