Arsenal - Reuse current inventory containers when importing invalid loadouts

[mrschick]

When merged this pull request will:

• When importing a stored loadout which has a uniform/vest/backpack that is missing or from an unloaded mod, retain the currently equipped container and fill it with available items from the stored loadout.

The use case for this change is missions in which the arsenal is restricted to clothing from a specific faction, but still contains all the usual medical and other specialized equipment for a given role. Previously, even if one had saved a very similar loadout, but with a different camo, it was impossible to load it without losing all clothing items. Forcing one to redo the entire loadout from scratch. With this change, it will be possible to import such a loadout, so that the numerous equipment items will be placed in already worn clothing of the specific faction, saving time.

If "" is given as a container, an error is thrown, therefore, filter out all unavailable/null containers

I realize that this was part of optimization work in #9316. Since it hinders this implementation, I removed that check. I have not found it causing errors when importing loadouts.