[php-src] Issue #23056: ext/session: "handler" part of warning broke in master

Issue: https://github.com/php/php-src/issues/23056
Author: ndossche

### Description

When executing the test `ext/session/tests/user_session_module/gh23043.phpt` has different output on master than on <=PHP8.5. It appears it previously printed the "handler" but now it no longer does. I got this diff when running the test on master:

```diff
     Warning: SessionHandler::write(): Session ID is too long or contains illegal characters. Only the A-Z, a-z, 0-9, "-", and "," characters are allowed in %s on line %d
     
005- Warning: session_write_close(): Failed to write session data using user defined save handler. (session.save_path: , handler: a::write) in %s on line %d
005+ Warning: session_write_close(): Failed to write session data using user defined save handler. (session.save_path: , handler: ) in ext/session/tests/user_session_module/gh23043.php on line 15
     string(0) ""
     
     Warning: SessionHandler::write(): Session ID is too long or contains illegal characters. Only the A-Z, a-z, 0-9, "-", and "," characters are allowed in Unknown on line 0
     
010- Warning: session_write_close(): Failed to write session data using user defined save handler. (session.save_path: , handler: a::write) in Unknown on line 0
010+ Warning: session_write_close(): Failed to write session data using user defined save handler. (session.save_path: , handler: ) in Unknown on line 0

```

Note how it previously said "handler: a::write" and now "handler: "

### PHP Version

```plain
master
```

### Operating System

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