Fix a crash in the gain selection part of the code in case there is a…

[vmarandon]

… missing packet

[codecov]

Codecov Report

Attention: Patch coverage is 13.60000% with 108 lines in your changes missing coverage. Please review.

Project coverage is 78.38%. Comparing base (815c090) to head (c0656ea).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ctapipe_io_nectarcam/__init__.py	13.60%	108 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #63       +/-   ##
===========================================
- Coverage   88.51%   78.38%   -10.13%     
===========================================
  Files           7        7               
  Lines         792      916      +124     
===========================================
+ Hits          701      718       +17     
- Misses         91      198      +107     

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