Calibration group hotfix

[kbwestfall]

Addresses #1823

The logic used to identify frames with calibration groups assumed there would always be at least one frame that had a single (numeric) calibration group. This fixes a failure caused when the pypeit file only contains frames that are set to all calibration groups.

[profxj]

boy we should refactor this someday :)

[debora-pe]

Looks good!

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 38.48%. Comparing base (b9dd4ce) to head (911e256).

Files	Patch %	Lines
pypeit/metadata.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1824   +/-   ##
========================================
  Coverage    38.48%   38.48%           
========================================
  Files          207      207           
  Lines        48151    48153    +2     
========================================
+ Hits         18532    18533    +1     
- Misses       29619    29620    +1     

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