nm-dispatcher: fix crash when parsing output dictionary

'stdout' is NULL when the script didn't write anything or failed.

Fixes the following crash detected by NMCI in test
'dispatcher_device_handler_dummy'.

  nm-dispatcher[936339]: g_strsplit: assertion 'string != NULL' failed

  build_result_options (nm-dispatcher)
  complete_request (nm-dispatcher)
  complete_script (nm-dispatcher)
  script_watch_cb (nm-dispatcher)
  g_child_watch_dispatch (libglib-2.0.so.0)
  g_main_dispatch (libglib-2.0.so.0)
  g_main_context_iterate (libglib-2.0.so.0)
  g_main_context_iteration (libglib-2.0.so.0)
  main (nm-dispatcher)
  __libc_start_main (libc.so.6)
  _start (nm-dispatcher)

Fixes: d72f26b875 ('dispatcher: read device-handler's stdout into a dictionary')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1889
(cherry picked from commit e5c2c5f1c2)

src/nm-dispatcher/nm-dispatcher.c

@@ -306,6 +306,9 @@ build_result_options(char *stdout)
     char                          *key;
     char                          *value;
 
+    if (!stdout)
+        return NULL;
+
     lines = g_strsplit(stdout, "\n", 65);
 
     for (i = 0; lines[i] && i < 64; i++) {