Fix crash on loading DICOM specs 2022b

[mrbean-bremen]

• crashed due to mishandled condition without values
• fixes IndexError parsing 2022b #15
• remove CI build for EOL Python 3.6 (no more available)

[codecov-commenter]

Codecov Report

Merging #16 (c0a8e6b) into main (33948a4) will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
+ Coverage   83.85%   84.24%   +0.39%     
==========================================
  Files          14       14              
  Lines        1344     1365      +21     
==========================================
+ Hits         1127     1150      +23     
+ Misses        217      215       -2     

Impacted Files	Coverage Δ
dicom_validator/spec_reader/condition.py	100.00% <100.00%> (ø)
dicom_validator/spec_reader/condition_parser.py	98.34% <100.00%> (+0.82%)	⬆️
dicom_validator/spec_reader/part3_reader.py	91.62% <0.00%> (+0.04%)	⬆️
dicom_validator/validator/iod_validator.py	92.52% <0.00%> (+0.10%)	⬆️
dicom_validator/spec_reader/spec_reader.py	90.90% <0.00%> (+0.34%)	⬆️
dicom_validator/spec_reader/edition_reader.py	75.71% <0.00%> (+1.21%)	⬆️

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