Fix warning on PHP8

[nanasess]

refs #1058
refs #1045

PHPUnit 及び E2Eテストで出る Warning の解消

setasign/fpdf で出る Warning は PR 済み(Setasign/FPDF#22)
[Standard input code] Warning(E_WARNING): Undefined array key "subsetted" on [/var/www/app/data/vendor/setasign/fpdf/fpdf.php(1658)] module/fpdi/japanese.php を修正

[codecov]

Codecov Report

Attention: Patch coverage is 70.07874% with 38 lines in your changes missing coverage. Please review.

Project coverage is 50.58%. Comparing base (27be918) to head (58c4a45).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
data/class/SC_Initial.php	0.00%	11 Missing ⚠️
data/class/helper/SC_Helper_HandleError.php	0.00%	6 Missing ⚠️
data/class/SC_Response.php	50.00%	5 Missing ⚠️
...ss/sessionfactory/SC_SessionFactory_UseRequest.php	0.00%	4 Missing ⚠️
data/class/util/GC_Utils.php	0.00%	4 Missing ⚠️
data/class/SC_ClassAutoloader.php	50.00%	2 Missing ⚠️
data/class/plugin/SC_Plugin_Util.php	0.00%	2 Missing ⚠️
...ass/sessionfactory/SC_SessionFactory_UseCookie.php	0.00%	2 Missing ⚠️
data/class/helper/SC_Helper_Session.php	0.00%	1 Missing ⚠️
data/class/plugin/SC_Plugin_Installer.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1077      +/-   ##
==========================================
- Coverage   51.67%   50.58%   -1.10%     
==========================================
  Files          80       80              
  Lines       10224    10257      +33     
==========================================
- Hits         5283     5188      -95     
- Misses       4941     5069     +128     

Flag	Coverage Δ
tests	50.58% <70.07%> (-1.10%)	⬇️

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