refactor: Explicit nullptr checks in Navigator

[andiwand]

There are a couple of implicit bool conversions in the Navigator which I propose to replace with explicit nullptr comparisons to improve readability.

[github-actions]

📊: Physics performance monitoring for 4fe47ec

Full contents

physmon summary

• ✅ CKF truth_smeared
• ✅ IVF truth_smeared
• ✅ AMVF truth_smeared
• ✅ Track Summary CKF truth_smeared
• ✅ Seeding truth_estimated
• ✅ CKF truth_estimated
• ✅ IVF truth_estimated
• ✅ AMVF truth_estimated
• ✅ Track Summary CKF truth_estimated
• ✅ Seeding seeded
• ✅ CKF seeded
• ✅ IVF seeded
• ✅ AMVF seeded
• ✅ AMVF (+grid seeder) seeded
• ✅ Track Summary CKF seeded
• ✅ Seeding orthogonal
• ✅ CKF orthogonal
• ✅ IVF orthogonal
• ✅ AMVF orthogonal
• ✅ Track Summary CKF orthogonal
• ✅ Ambisolver seeded
• ✅ Ambisolver orthogonal
• ✅ Seeding ttbar
• ✅ CKF ttbar
• ✅ Ambisolver
• ✅ Track Summary CKF ttbar
• ✅ AMVF ttbar
• ✅ AMVF (+grid seeder) ttbar
• ✅ Truth tracking (GSF)
• ✅ Truth tracking
• ✅ Truth tracking (GX2F)
• ✅ Particles fatras
• ✅ Particles geant4

[AJPfleger]

lgtm ＼（＾○＾）人（＾○＾）／

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 47.61%. Comparing base (d7aa303) to head (4fe47ec).
Report is 249 commits behind head on main.

Files	Patch %	Lines
Core/include/Acts/Propagator/Navigator.hpp	83.33%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3218   +/-   ##
=======================================
  Coverage   47.61%   47.61%           
=======================================
  Files         507      507           
  Lines       29163    29167    +4     
  Branches    13996    14000    +4     
=======================================
+ Hits        13885    13889    +4     
  Misses       5260     5260           
  Partials    10018    10018           

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[acts-project-service]

🔴 Athena integration test results [538a1fa]

🔴 Some tests have failed!

Please investigate the pipeline!

status	job	report
🟢	run_unit_tests
🔴	test_ActsCheckObjectCountsWorkflow
🟢	test_ActsEFTrackFit
🟢	test_ActsPersistifySeeds
🟢	test_ActsBenchmarkWithSpot
🟢	test_ActsAnalogueClustering
🟢	test_ActsWorkflowHeavyIons
🟢	test_ActsWorkflowFastTracking
🟢	test_ActsWorkflowCached
🟢	test_ActsWorkflow
🟢	test_ActsValidateAmbiguityResolution
🟢	test_ActsValidateResolvedTracks
🟢	test_ActsValidateTracks
🟢	test_ActsValidateActsCoreSpacePoints
🟢	test_ActsValidateActsSpacePoints
🟢	test_ActsValidateSeeds
🟢	test_ActsValidateOrthogonalSeeds
🟢	test_ActsValidateClusters
🟢	test_ActsPersistifyEDM
🟢	test_ActsGSFRefitting
🟢	test_ActsKfRefitting
🟢	test_ActsExtrapolationAlgTest
🟢	test_ActsITkTest
🟢	run_workflow_tests_run4_mc
🟢	run_workflow_tests_run2_mc
🟢	run_workflow_tests_run2_data
🟢	run_workflow_tests_run3_mc
🟢	run_workflow_tests_run3_data
🟢	run_art_test: test_data18_13TeV_1000evt
🟢	run_art_test: test_ttbarPU40_reco