XSH-867: Use None in DataFrame index/column names for missing headers

[torchbear]

• because pandas.io.formats.style.Styler.to_excel expects empty headers with None otherwise it takes the value and prints it which is not expected for empty string in this case

[ghost]

Build succeeded (check pipeline).

[codecov-commenter]

Codecov Report

Merging #169 (409fde2) into master (9aedb61) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   92.01%   92.02%   +0.01%     
==========================================
  Files          65       65              
  Lines        4231     4238       +7     
==========================================
+ Hits         3893     3900       +7     
  Misses        338      338              

Impacted Files	Coverage Δ
...ooddata-pandas/gooddata_pandas/result_convertor.py	94.47% <100.00%> (ø)
gooddata-sdk/gooddata_sdk/catalog/base.py	97.29% <100.00%> (+0.07%)	⬆️
...dk/catalog/data_source/entity_model/data_source.py	99.20% <100.00%> (+0.04%)	⬆️

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Build succeeded (check pipeline).

[yenkins]

Sonar scan result

More detail, see in https://sonarqube-gate.intgdc.com/dashboard?id=gooddata-python-sdk-gate-PR169

To scan for vulnerabilities in dependencies and run unit tests (to get coverage report in sonar) please comment your PR with 'extended check sonar'.

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Build succeeded (gate pipeline).

• https://checklist.intgdc.com/job/compliance/job/gooddata-python-sdk-fossa-scanning-zuul/5/ : SUCCESS in 3m 17s (non-voting)
• https://checklist.intgdc.com/job/compliance/job/gooddata-python-sdk-sonarqube-scan-zuul-gate/155/ : SUCCESS in 1m 35s
• https://checklist.intgdc.com/job/compliance/job/gooddata-python-sdk-merge-compliance-check/157/ : SUCCESS in 39s