Fix code quality issues reported by PHPStan

Problem/Motivation

Currently there are some code errors reported in PHPStan CI job:

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  Line   tests/src/Kernel/Entity/EntityViewModeDeleteFormHelperTest.php  
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  136    No error to ignore is reported on line 136.                     
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 ------ ------------------------------------------------------------------------ 
  Line   tests/src/Kernel/Plugin/Field/FieldType/ViewModeSwitchItemListTest.php  
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  159    No error to ignore is reported on line 159.                             
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  Line   view_mode_switch.install                    
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  66     No error to ignore is reported on line 66.  
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 [ERROR] Found 3 errors       

Steps to reproduce

• Check output of recent PHPStan job(s) in CI

Proposed resolution

• Fix code quality issues

Remaining tasks

• Create patch/MR to fix the code quality issues

User interface changes

n/a

API changes

n/a

Data model changes

n/a