Fix precomputation for SERVICE in JOIN operation

[UNEXENU]

This fixes #1659, an issue introduced in #1556 that made the precomputation of a SERVICE sibling in JOIN operations impossible.

[sparql-conformance]

Conformance check passed ✅

No test result changes.

Details: https://qlever.cs.uni-freiburg.de/sparql-conformance-ui?cur=5be697ce4a1bdf6f902dcdbd5111556965562eea&prev=ec806f0091babfdcb57b20189c7656337b3f7a71

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.56%. Comparing base (ec806f0) to head (5be697c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1661      +/-   ##
==========================================
- Coverage   89.58%   89.56%   -0.02%     
==========================================
  Files         381      381              
  Lines       36813    36813              
  Branches     4173     4173              
==========================================
- Hits        32978    32972       -6     
- Misses       2523     2524       +1     
- Partials     1312     1317       +5     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[joka921]

Thank you very much for this fix, @hannahbast has tried it out and confirmed, that it works as expected.