block containers: fix parsing of trailing whitespace

[biojppm]

fixes #475

[codecov]

Codecov Report

Attention: Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.27%. Comparing base (9b8df62) to head (17000de).

Files with missing lines	Patch %	Lines
src/c4/yml/parse_engine.def.hpp	97.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   97.28%   97.27%   -0.01%     
==========================================
  Files          34       34              
  Lines       11270    11283      +13     
==========================================
+ Hits        10964    10976      +12     
- Misses        306      307       +1     

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