refactor: RedisHandler ttl() calls

[kenjis]

Description

• fix PHPStan 1.10.41 errors
  • https://github.com/phpstan/phpstan/releases/tag/1.10.41

Error: Comparison operation ">" between (int|Redis|false) and 0 results in an error.
Error: Ignored error pattern #^Only booleans are allowed in a negated boolean, int given\.$# in path /home/runner/work/CodeIgniter4/CodeIgniter4/system/Session/Handlers/RedisHandler.php was not matched in reported errors.
Error: Comparison operation ">" between (int|Redis|false) and 0 results in an error.
 ------ --------------------------------------------------------------------- 
  Line   system/Cache/Handlers/RedisHandler.php                               
 ------ --------------------------------------------------------------------- 
  250    Comparison operation ">" between (int|Redis|false) and 0 results in  
         an error.                                                            
 ------ --------------------------------------------------------------------- 

 ------ -------------------------------------------------------------------------------------- 
  Line   system/Session/Handlers/RedisHandler.php                                              
 ------ -------------------------------------------------------------------------------------- 
         Ignored error pattern #^Only booleans are allowed in a negated                        
         boolean, int given\.$# in path                                                        
         /home/runner/work/CodeIgniter4/CodeIgniter4/system/Session/Handlers/RedisHandler.php  
         was not matched in reported errors.                                                   
  296    Comparison operation ">" between (int|Redis|false) and 0 results in                   
         an error.                                                                             
 ------ -------------------------------------------------------------------------------------- 

https://github.com/codeigniter4/CodeIgniter4/actions/runs/6763592529/job/18380895677

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

[kenjis]

PHPStan now reports errors. Please review.

[kenjis]

Thank you! @paulbalandan