受注編集画面で数値項目に空文字を入力された場合にシステムエラーになるのを修正

[nanasess]

refs #829
とり急ぎ応急処置。数量及び税率が空文字の場合は0にキャストする。
根治対応は #836 で行う

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.94%. Comparing base (6760492) to head (5d96e1d).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1129   +/-   ##
=======================================
  Coverage   49.94%   49.94%           
=======================================
  Files          82       82           
  Lines       10480    10480           
=======================================
  Hits         5234     5234           
  Misses       5246     5246           

Flag	Coverage Δ
tests	49.94% <ø> (ø)

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[seasoftjapan]

数量を空文字としても、登録できた。
再表示時は 0 となっていた。