cmake: Tolerate nullptr from uv_default_loop

`uv_default_loop()` can return `nullptr` when running on a Linux kernel
configured without `CONFIG_EVENTFD`.

Fixes: #20899

Source/cmSystemTools.cxx

@@ -823,7 +823,9 @@ void cmSystemTools::InitializeLibUV()
   // Perform libuv one-time initialization now, and then un-do its
   // global _fmode setting so that using libuv does not change the
   // default file text/binary mode.  See libuv issue 840.
-  uv_loop_close(uv_default_loop());
+  if (uv_loop_t* loop = uv_default_loop()) {
+    uv_loop_close(loop);
+  }
 #  ifdef _MSC_VER
   _set_fmode(_O_TEXT);
 #  else

Source/cmakemain.cxx

@@ -717,6 +717,8 @@ int main(int ac, char const* const* av)
 #ifndef CMAKE_BOOTSTRAP
   cmDynamicLoader::FlushCache();
 #endif
-  uv_loop_close(uv_default_loop());
+  if (uv_loop_t* loop = uv_default_loop()) {
+    uv_loop_close(loop);
+  }
   return ret;
 }