Allow creation of empty Tracks object

[cmalinmayor]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.38%. Comparing base (066b558) to head (3088e1e).
Report is 13 commits behind head on v0.2-dev.

Files with missing lines	Patch %	Lines
finn/layers/tracks/tracks.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           v0.2-dev      #16      +/-   ##
============================================
+ Coverage     89.30%   89.38%   +0.07%     
============================================
  Files           651      651              
  Lines         59932    59952      +20     
============================================
+ Hits          53523    53587      +64     
+ Misses         6409     6365      -44     

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