Missing texture when an item requests a nonexistent texture for the camera overlay

Relates to MC-276671 and MC-276673.

The bug

If an item which can be equipped defines a camera overlay, and points to a texture file which does not exist, the missing texture will be displayed.

How to reproduce

1. /give @s slime_block[equippable={slot:"head",equip_sound:"block.sculk_shrieker.shriek",dispensable:true,camera_overlay:thistexturedoesnotexist}]

2. Equip

Expected results

Since the texture specified does not exist, there would be no camera overlay at all.

Actual results

Missing Texture Jumpscare

How to fix

If a nonexistent texture is specified, do not render an overlay at all.