Do not add group values of zero to group lists. (#8751)

This fixes an issue where when the engine looked up groups (for example,
in ABM node names), NodeDefManager's m_group_to_items would contain nodes
with a group value of zero, resulting in nodes with flammable = 0 being
burned by a fire mod with a group:flammable checking ABM.

It brings consistency to the behaviour described in the api
documentation, where zero and nil groups should be the same.

src/script/common/c_content.cpp

@@ -1535,6 +1535,8 @@ void read_groups(lua_State *L, int index, ItemGroupList &result)
 		// key at index -2 and value at index -1
 		std::string name = luaL_checkstring(L, -2);
 		int rating = luaL_checkinteger(L, -1);
+		// zero rating indicates not in the group
+		if (rating != 0)
 			result[name] = rating;
 		// removes value, keeps key for next iteration
 		lua_pop(L, 1);