fix: Ajax request makes the session expire when app.sessionExpiration is set to 0

[kenjis]

Description
Fixes #5688

• When setting in .env, all values are string.

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]

Should we fix DotEnv to convert variable types?
No, we use not only DotEnv but also environment variables which are all strings.

[kenjis]

Should we fix BaseConfig::initEnvValue() to convert variable types?
Ref #5691

[kenjis]

#5779 was merged.