fix(data_frame): parsing empty query result value as numpy.NaN

[bednar]

Closes #550

Proposed Changes

The empty values in query results from serialization into Pandas DataFrame are returned as numpy.NaN values instead of '' - empty string.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8fa5146) 90.29% compared to head (fb19fe9) 90.30%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #584   +/-   ##
=======================================
  Coverage   90.29%   90.30%           
=======================================
  Files          39       39           
  Lines        3452     3455    +3     
=======================================
+ Hits         3117     3120    +3     
  Misses        335      335           

Impacted Files	Coverage Δ
influxdb_client/client/flux_csv_parser.py	98.42% <100.00%> (+0.01%)	⬆️

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