Fix null tools

[mugling]

Inspired by #17025 and fixes #16751. Tried submitting a PR for @Coolthulhu's repository but can't get it to work.

• a650e6d is @Coolthulhu's migration for existing bugged saves
• d372d16 stops items with integral magazines spawning with a null magazine when with_ammo
• 4fe7f75 incidentally increases the robustness of item::ammo_set

[Coolthulhu]

Would be better if it described the bug, such as "Fixes bugged saves with tools containing nulls"

[Coolthulhu]

It may be worth adding a is_null check in the emplace function (and debugmsg on null) to prevent it happening later on.

[mugling]

@Coolthulhu I did consider this. Is it likely to prove a performance concern?

[Coolthulhu]

No, is_null check is at worst a string comparison, emplace( item ) is an order of magnitude more expensive.

[mugling]

@Coolthulhu see d001e67. We can't back out of adding a null item due to the return of a reference but we can add a debugmsg