[FIX] Writers: Store nans in StringVariable as empty strings instead of 'nan'

[janezd]

Issue

Fixes #6527.

Description of changes

The function that creates a string representation for storing values in file didn't expect that a StringVariable can be np.nan. Now it does.

Our handling of StringVariable unknowns is a mess; the Table widget will apparently sometimes show nan. This must be solved in general. The present PR only fixes the crash in saving of Excel files.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #6670 (047400d) into master (ef3465d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6670   +/-   ##
=======================================
  Coverage   88.10%   88.11%           
=======================================
  Files         321      321           
  Lines       69950    69951    +1     
=======================================
+ Hits        61631    61634    +3     
+ Misses       8319     8317    -2     

[markotoplak]

I think this should be made specific to CSV writer. Any reason you did not make it that way?

Tab writers work well as they are and this fix would there only introduce new ambiguities.

[markotoplak]

Ignore the previous comment. I need to check what did it do in write/read combination. If the old .tab behavior was already broken, I have no objections.

[markotoplak]

Our tab saving was also broken, because it saved a string nan in such cases. Therefore, merging.