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)));

test/functional/lua/thread_spec.lua

@@ -19,6 +19,26 @@ describe('thread', function()
     screen = Screen.new(50, 10)
   end)
 
+  it('handle non-string error', function()
+    exec_lua [[
+      local thread = vim.uv.new_thread(function()
+        error()
+      end)
+      vim.uv.thread_join(thread)
+    ]]
+
+    screen:expect([[
+                                                        |
+      {1:~                                                 }|*5
+      {3:                                                  }|
+      {9:Error in luv thread:}                              |
+      {9:[NULL]}                                            |
+      {6:Press ENTER or type command to continue}^           |
+    ]])
+    feed('<cr>')
+    assert_alive()
+  end)
+
   it('entry func is executed in protected mode', function()
     exec_lua [[
       local thread = vim.uv.new_thread(function()