fix(pcre): preserve unmatched groups in Preg::match_all*()

PHP's Preg::matchAll() and matchAllWithOffsets() always set
PREG_UNMATCHED_AS_NULL, so a non-participating group is `null` and its
offset is -1. The Rust wrappers collapsed those to "" and 0, so callers
could not tell a group that did not participate from one that matched an
empty string at offset 0, and the offset value matched no PHP mode at
all.

Hand the shim's representation through unchanged and let each caller
mirror what the PHP original does with it: `isset()` and `(string)`
casts stay lenient, while `assert(is_string(...))` and the
*StrictGroups() variants become `expect()`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions