Return None if response is empty

[edkry]

Required: If there is no data for a table with specific parameters, it should return None.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.80%. Comparing base (8d5e467) to head (3190fa7).
⚠️ Report is 433 commits behind head on main.

Files with missing lines	Patch %	Lines
sdmx/client.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
- Coverage   98.86%   97.80%   -1.07%     
==========================================
  Files          94       94              
  Lines        7431     7433       +2     
==========================================
- Hits         7347     7270      -77     
- Misses         84      163      +79     

Files with missing lines	Coverage Δ
sdmx/client.py	93.51% <50.00%> (-5.40%)	⬇️

... and 17 files with indirect coverage changes

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