[php-src] PR #25252: Fix GH-25249: Fibers start with error_reporting = 0 when the directive is unset

Pull Request: https://github.com/php/php-src/pull/25252 Author: rafamolina1 ## Summary - When error_reporting is unset, OnUpdateErrorReporting() stores E_ALL in EG(error_reporting), but the INI value stays NULL. zend_fiber_execute() reads that entry with zend_ini_long(), gets 0, and the fiber drops warnings. This is what php -n and the official images without a php.ini do. GH-23930 removed the old E_ALL fallback. - Restore the fallback only when zend_ini_str_ex() returns NULL. zend_ini_str() cannot be used here: it turns that NULL into an empty string. An explicit 0 or an empty value stays 0. - The fiber still reads the INI entry rather than EG(error_reporting), so the @ operator outside the fiber does not leak in. Fixes GH-25249. ## Test plan - [x] Zend/tests/fibers/gh25249.phpt runs a child with php -n and expects 30719 plus both undefined-variable warnings - [x] gh23921.phpt and silence-operator-outside-fiber-should-no-widen-error-reporting.phpt pass - [x] Zend/tests/fibers passes (121 passed, 2 skipped for missing pcntl and posix) - [x] The same three tests pass with opcache.jit=function and opcache.jit=tracing - [x] php -n prints 30719 in the main script and in the fiber; -d error_reporting=0 and an empty directive stay 0 - [x] ASAN, debug, and ZTS: Zend/tests/fibers passes (118 passed, 5 skipped)