fix(lua): prevent SIGSEGV when lua error is NULL in libuv_worker

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.

src/nvim/lua/executor.c

@@ -276,10 +276,9 @@ static int nlua_luv_thread_common_cfpcall(lua_State *lstate, int nargs, int nres
 #endif
     }
     const char *error = lua_tostring(lstate, -1);
-
     loop_schedule_deferred(&main_loop,
                            event_create(nlua_luv_error_event,
-                                        xstrdup(error),
+                                        error != NULL ? xstrdup(error) : NULL,
                                         (void *)(intptr_t)(is_callback
                                                            ? kThreadCallback
                                                            : kThread)));