Make output for results with zero columns standard-compliant

[hannahbast]

For queries with zero columns (e.g., SELECT * WHERE { <s> <p> <o> }), the results were not fully standard-compliant. In particular, only the empty header row was output for text/tab-separated-values and text/csv, but never any additional empty rows. This is now fixed. Fixes #1745

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.97%. Comparing base (f2562fe) to head (953a7c8).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/engine/ExportQueryExecutionTrees.cpp	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1749      +/-   ##
==========================================
+ Coverage   89.95%   89.97%   +0.02%     
==========================================
  Files         395      395              
  Lines       37651    37654       +3     
  Branches     4234     4234              
==========================================
+ Hits        33869    33880      +11     
+ Misses       2487     2478       -9     
- Partials     1295     1296       +1     

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[sparql-conformance]

Conformance check passed ✅

No test result changes.

Details: https://qlever.cs.uni-freiburg.de/sparql-conformance-ui?cur=953a7c8095fa992b33753afb18e8da540569d0b5&prev=f2562fe2ca1da7e1704aa2a5b0473be08e5f296c

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[joka921]

Thank you very much

[joka921]

For queries with zero columns (e.g., SELECT * WHERE { <s> <p> <o> }), the results were not fully standard-compliant. In particular, only the empty header row was output for text/tab-separated-values and text/csv, but never any additional empty rows. This is now fixed. Fixes #1745