…legacy blocks with air in //generate (#2901)

* In //g, allow overriding non-legacy blocks with air

Previously, it was impossible to override a non-legacy block (i.e. one that was introduced after the switch to string IDs) with air.
This was due to the fact that non-legacy blocks and air had the same type id (0).
This change gives them separate type ids, making it possible to override non-legacy blocks with air.

* Make query functions return type/data values of -1 for non-legacy blocks

This matches the new behaviour of the type/data variables in //generate