入力が無い場合にも MIN_LENGTH_CHECK がエラー判定となってしまうのを修正

[nanasess]

Fixes #115

MIN_LENGTH_CHECK は EC-CUBE本体では使用されていない模様

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.61%. Comparing base (aab3658) to head (a4ab1b2).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1112      +/-   ##
==========================================
+ Coverage   50.60%   50.61%   +0.01%     
==========================================
  Files          80       80              
  Lines       10258    10261       +3     
==========================================
+ Hits         5191     5194       +3     
  Misses       5067     5067              

Flag	Coverage Δ
tests	50.61% <100.00%> (+0.01%)	⬆️

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