Fix inability to change metadata using stack:to_table() (#5547)

src/script/lua_api/l_item.cpp

@@ -234,6 +234,8 @@ int LuaItemStack::l_to_table(lua_State *L)
 		for (StringMap::const_iterator it = fields.begin();
 				it != fields.end(); ++it) {
 			const std::string &name = it->first;
+			if (name.empty())
+				continue;
 			const std::string &value = it->second;
 			lua_pushlstring(L, name.c_str(), name.size());
 			lua_pushlstring(L, value.c_str(), value.size());