Fix handling of non-existent reflectance bands in 'viirs_l1b' reader

[djhoese]

The VIIRS L1b format was changed a couple years ago to save space by only including reflectance band when there is day data in the scene. The variables in these cases do not exist in the files. This resulted in two problems:

1. Trying to load a reflectance band would produce an unexpected error related to trying to treat an integer shape (default return value to handle scalar variables) as a tuple.
2. Listing the reflectance bands as available in Scene.available_dataset_ids/names.

This PR fixes both those cases.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2100 (d8ed23a) into main (2e1d845) will increase coverage by 0.02%.
The diff coverage is 96.07%.

@@            Coverage Diff             @@
##             main    #2100      +/-   ##
==========================================
+ Coverage   93.90%   93.92%   +0.02%     
==========================================
  Files         283      283              
  Lines       42615    42715     +100     
==========================================
+ Hits        40017    40120     +103     
+ Misses       2598     2595       -3     

Flag	Coverage Δ
behaviourtests	4.71% <0.00%> (-0.01%)	⬇️
unittests	94.48% <96.07%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_viirs_l1b.py	98.93% <93.93%> (+1.66%)	⬆️
satpy/readers/viirs_l1b.py	96.15% <100.00%> (+0.37%)	⬆️
satpy/readers/cmsaf_claas2.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_cmsaf_claas.py	100.00% <0.00%> (ø)
satpy/tests/test_scene.py	99.47% <0.00%> (+<0.01%)	⬆️
satpy/scene.py	93.32% <0.00%> (+0.20%)	⬆️

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[coveralls]

Coverage increased (+0.01%) to 94.421% when pulling d8ed23a on djhoese:bugfix-viirs-l1b-available into e5a71d5 on pytroll:main.

[mraspaud]

LGTM, just a small improvement suggestion inline.