Matchbox fix

[kevingranade]

Summary

SUMMARY: None

Purpose of change

Fixes #39043
We've had a very intermittent error for who knows how long when matchbooks are spawned.
It turns out this happens when an item_group has a non-zero with_ammo field and a child item modifier of it is a tool with default charges and a null ammo type.

Describe the solution

The issue turns out to be a mismatch between null strings produced by item::default_ammo() and expected by item::ammo_set().

Describe alternatives you've considered

A more complete fix to this would be to make itype_id or possibly just the ammo string a formal type instead of using std::string, but that's a pretty huge change I don't have the stomach to tackle right now.
A slightly more complete fix that I might consider soon is auditing the uses of "null" vs "NULL" in the item code and data to eliminate errors of this kind, but we need to be careful because remaining mismatches can cause further errors.

Testing

The recently added mx_minefield test was triggering this fairly frequently by simply generating a map extra that contained an affected item group. I was able to isolate the cause and add a unit test for it.