Fix realpath('') empty path — resolve cwd like Zend (#10257)

[PurHur]

Summary

• Treat realpath('') as realpath('.') per php-src ext/standard/basic_functions.c php_realpath() empty-path → VCWD_GETCWD behavior
• Normalize empty path in VmString::realpath(), VmStatNative::realpath(), and JitRealpath LLVM lowering
• Add compliance guard test/compliance/cases/stdlib/realpath_empty_string.phpt and issue repro script

php-src reference

• ext/standard/basic_functions.c — php_realpath() empty path uses cwd

PHP implementation

• ext/standard/VmString.php — VM SSOT
• ext/standard/VmStatNative.php — libc FFI path
• ext/standard/JitRealpath.php — JIT empty-string → . select

Closes #10257

Test plan

php bin/vm.php test/repro/maintainer_gap_realpath_empty.php
# '/app/var/workspaces/.../repo'  (cwd string, not false)

php bin/vm.php test/compliance/cases/stdlib/realpath_empty_string.phpt
# ok

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