Fix crash on parsing of zero-prefixed RCNs

[jodal]

This PR fixes a crash on parsing of RCNs that are zero-padded to a GTIN-14.

Also, add tests of:

• German variable measure check digit error
• German variable measure zeroing

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (7dbd304) compared to base (912920f).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 7dbd304 differs from pull request most recent head 29c8426. Consider uploading reports for the commit 29c8426 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #150   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          851       853    +2     
  Branches       127       127           
=========================================
+ Hits           851       853    +2     

Impacted Files	Coverage Δ
src/biip/gtin/_rcn.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.