Stdlib: coerce null to empty string for typed string builtins (#18483)

[PurHur]

Summary

• Restores Zend Z_PARAM_STR null→"" coercion for addslashes, stripslashes, htmlspecialchars, htmlentities, strip_tags, wordwrap, and addcslashes after the Stdlib: null operand coercion parity batch (#18369–#18378) #18385 regression reintroduced rejectNullString / coerceTypedStringBuiltinArg paths.
• VM: route $string through InternalStrictArg::resolveCoercibleStringArg (respects caller strict_types).
• JIT: switch from lowerTypedString / rejectNullString to lowerStrictOrCoercible.
• php-src reference: ext/standard/string.c, ext/standard/html.c

Verification

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/string_builtin_null.php'
# htmlspecialchars: uncaught
# htmlentities: uncaught
# strtolower: uncaught
# addcslashes: uncaught
# trim: uncaught
# strip_tags: uncaught

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter string_builtin_null'
# OK (VM + JIT compliance)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter addslashes_scalar_coercion'
# VM OK; JIT/AOT pre-existing infra issues on this host

Closes #18483

Made with Cursor

[PurHur]

Duplicate of merged #18486 — closing.