Filtering on key only if key exists using overpass query language

I want to match bridges which are considered metallic or unknown.

I tried this:

area[name="Australia"]->.a;
way(area.a)
[man_made=bridge]
[bridge!=viaduct]
["bridge:structure"!=humpback]
[material~metal]
[material~steel]
[material~reinforced_concrete]
["bridge:material"~steel]
["bridge:material"~metal];
out geom;

But it this ignores bridges that don't have the material or "bridge:material" key. Looking at the overpass docs doesn't mention anyway to match on a key only if that key exists.