Equippable items with invalid specified equipment models do not use the missing texture

Testing discovered MC-276770 and MC-276771.

The bug

If an item which can be equipped is specified via components to point to an equipment model that does not exist, the missing texture will not be used when it is equipped.

As per the resolutions of MC-276671 and MC-276673, this may not be intended behaviour.

How to reproduce

1. /give @s stick[equippable={slot:"chest",equip_sound:"entity.ghast.hurt",model:leather}]

2. /give @s stick[equippable={slot:"chest",equip_sound:"entity.ghast.hurt",model:nonexistent_model}]

3. Equip each and observe

Expected results

As leather is a valid model, the first item would display the leather armor texture. As nonexistent_model does not exist, there would be a missing texture displayed.

Actual results

The first does in fact happen, however for the invalid model, there is no missing texture.

How to fix

Either display missing textures here, or fix MC-276671 and MC-276673 by making them fall back to the default textures/models.