Stdlib: strcmp()/strncmp() null operands — empty-string coercion (#11261)

[PurHur]

Summary

• strcmp() / strncmp() coerce null to "" without strict_types, matching php-src ext/standard/string.c.
• Under strict_types (including via closures), null/bool operands still TypeError (Stdlib: strnatcmp()/strcoll()/strncmp()/version_compare() — null/bool operands must TypeError (ext/standard/string.c) #10908 unchanged).
• Fix VmString::strncmp() padding when one operand is empty (returns -1/1 not byte diff).
• JIT: switch from lowerRequiredString to JitStringBuiltinArg::lower().

Closes #11261

Verification

./script/docker-exec.sh -- bash -lc '
  source script/php-env.sh
  php test/repro/maintainer_gap_strcmp_null_coerce.php
  php bin/vm.php test/repro/maintainer_gap_strcmp_null_coerce.php
  php bin/vm.php test/compliance/cases/stdlib/strcmp_null_coerce.phpt
  php bin/vm.php test/compliance/cases/stdlib/string_compare_null_scalar_typeerror.phpt
  php bin/compile.php -l test/repro/maintainer_gap_strcmp_null_coerce.php
'

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