fix: Let Root writers accept empty Trajectories object

[benjaminhuth]

The Root Writers crash when they encounter an empty Trajectories object (e.g., from a failed fit) which should be fixed by this PR.

One thing that is not optimal is that we seem not to test a case with failing tracks, but I'm not sure how to setup such a test in the best way...

[codecov]

Codecov Report

Merging #1864 (5768cd8) into main (5f2cad4) will not change coverage.
The diff coverage is n/a.

❗ Current head 5768cd8 differs from pull request most recent head d9ff252. Consider uploading reports for the commit d9ff252 to get more accurate results

@@           Coverage Diff           @@
##             main    #1864   +/-   ##
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  Coverage   49.50%   49.50%           
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  Files         407      407           
  Lines       22646    22646           
  Branches    10334    10334           
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  Hits        11210    11210           
  Misses       4247     4247           
  Partials     7189     7189           

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[github-actions]

📊 Physics performance monitoring for d9ff252

Full report
Seeding: seeded, truth estimated, orthogonal
CKF: seeded, truth smeared, truth estimated, orthogonal
IVF: seeded, truth smeared, truth estimated, orthogonal
Ambiguity resolution: seeded, orthogonal
Truth tracking
Truth tracking (GSF)

Vertexing

Vertexing vs. mu

IVF seeded

IVF truth_smeared

IVF truth_estimated

IVF orthogonal

Seeding

Seeding seeded

Seeding truth_estimated

Seeding orthogonal

CKF

CKF seeded

CKF truth_smeared

CKF truth_estimated

CKF orthogonal

Ambiguity resolution

seeded

Truth tracking (Kalman Filter)

Truth tracking

Truth tracking (GSF)

Truth tracking