plugins/python/python_plugin_policy: fix validate() call

When calling validate() python function, TypeError exception was thrown
("argument list must be a tuple"), because the call does not have
arguments, and python does not accept empty tuple for execution. NULL
must be used instead, which was handled as argument construction failure
previously.

plugins/python/python_plugin_common.c

@@ -342,7 +342,9 @@ python_plugin_api_call(struct PluginContext *plugin_ctx, const char *func_name,
 {
     debug_decl(python_plugin_api_call, PYTHON_DEBUG_PY_CALLS);
 
-    if (py_args == NULL) {
+    // Note: call fails if py_args is an empty tuple. Passing no arguments works passing NULL
+    // instead. So having such must be handled as valid. (See policy_plugin.validate())
+    if (py_args == NULL && PyErr_Occurred()) {
         py_sudo_log(SUDO_CONV_ERROR_MSG, "Failed to build arguments for python plugin API call '%s'\n", func_name);
         py_log_last_error(NULL);
         debug_return_ptr(NULL);

plugins/python/python_plugin_policy.c

@@ -200,8 +200,7 @@ int
 python_plugin_policy_validate(void)
 {
     debug_decl(python_plugin_policy_validate, PYTHON_DEBUG_CALLBACKS);
-    debug_return_int(python_plugin_api_rc_call(&plugin_ctx, CALLBACK_PYNAME(validate),
-                                               Py_BuildValue("")));
+    debug_return_int(python_plugin_api_rc_call(&plugin_ctx, CALLBACK_PYNAME(validate), NULL));
 }
 
 void