[4.5] refactor: use local variables in Model

[kenjis]

Description
To fix PHPStan error. Also, using a local variable is a bit faster than method calling twice.

  ------ -------------------------------------------- 
  Line   system/Model.php                            
 ------ -------------------------------------------- 
  211    Variable $returnType might not be defined.  
  213    Variable $returnType might not be defined.  
  275    Variable $returnType might not be defined.  
  278    Variable $returnType might not be defined.  
  316    Variable $returnType might not be defined.  
  318    Variable $returnType might not be defined.  
 ------ -------------------------------------------- 

https://github.com/codeigniter4/CodeIgniter4/actions/runs/7982852609/job/21797025530

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide