Make LightmapGIData::_set_user_data a proper setter instead of an additive operation

scene/3d/lightmap_gi.cpp

@@ -81,9 +81,8 @@ void LightmapGIData::clear_users() {
 }
 
 void LightmapGIData::_set_user_data(const Array &p_data) {
-	ERR_FAIL_COND(p_data.is_empty());
 	ERR_FAIL_COND((p_data.size() % 4) != 0);
-
+	users.clear();
 	for (int i = 0; i < p_data.size(); i += 4) {
 		add_user(p_data[i + 0], p_data[i + 1], p_data[i + 2], p_data[i + 3]);
 	}