Problem:
Calling `xstrdup` with a NULL pointer causes a SIGSEGV if `lua_tostring` returns
NULL in `nlua_luv_thread_common_cfpcall`.
Crash stack trace:
- `_platform_strlen` → `xstrdup` (memory.c:469)
- `nlua_luv_thread_common_cfpcall` (executor.c:281)
Solution:
Check if `lua_tostring` returns NULL and pass NULL to `event_create` to avoid the crash.