fix(config): add null coalescing fallback in getValueBool before strtolower

[skjnldsv]

• Relates to: fix(UserConfig): cast getTypedValue() result to string in getValueBool() #59646

Context

Despite having the patch from #59646, a customer is still experiencing the following:

 "url": "/index.php/login",
 "scriptName": "/index.php",
 "message": "strtolower(): Argument #1 ($string) must be of type string, string given in file '/var/www/lajen/lib/private/Config/UserConfig.php' line 688",
 "userAgent": "Mozilla/5.0 (X11; Linux x86_64; rv:149.0) Gecko/20100101 Firefox/149.0",
 "version": "33.0.3.3",

Summary

Followup to #59646: both AppConfig::getValueBool() and UserConfig::getValueBool() pass the return value of getTypedValue() directly to strtolower(). Although the declared return type is string, the same OPcache/PHP bug that motivated the (string) cast in UserConfig can cause a null to slip through at runtime.

• Added ?? 'false' null coalescing fallback in UserConfig::getValueBool()
• Added (string) cast, OPcache comment, and ?? 'false' fallback in AppConfig::getValueBool(), aligning it with the pattern introduced for UserConfig in fix(UserConfig): cast getTypedValue() result to string in getValueBool() #59646

TODO

• No further changes needed — purely defensive null guard

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation has been updated or is not required
• Backports requested where applicable
• Labels added where applicable (bug, 3. to review)
• Milestone added for target branch/version

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[skjnldsv]

/backport to stable34

[skjnldsv]

/backport to stable33

[skjnldsv]

/backport to stable32