お問い合わせフォームの入力チェックでシステムエラーになるのを修正

[nanasess]

PHP8において、お問い合わせフォームのお問い合わせ覧を空欄で確認画面へ進むとシステムエラーになるのを修正

以下開発コミュニティからの報告
https://xoops.ec-cube.net/modules/newbb/viewtopic.php?topic_id=26712&forum=8&post_id=106064#forumpost106064

[codecov-commenter]

Codecov Report

Merging #536 (3ff4360) into master (0c0c25a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #536   +/-   ##
=======================================
  Coverage   54.53%   54.53%           
=======================================
  Files          76       76           
  Lines        9065     9065           
=======================================
  Hits         4944     4944           
  Misses       4121     4121           

Flag	Coverage Δ
tests	54.53% <ø> (ø)

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[chihiro-adachi]

@nanasess
ありがとうございます。マージしました。