Severe lag when upgrading 1.20.4 worlds

Upgrading worlds from 1.20.4 to 1.20.5-pre4 can cause significant lag. This appears to be caused by suspicious blocks (sand and gravel) and likely others, that previously stored minecraft:air item to represent no item, but since 24w09a? expect the item field to not be present instead.

The attached test world contains a big suspicious sand cube, which causes a lot of log spam. But the same can be observed in regular worlds, where builders or world generation make use a lot of these blocks for texturing purposes.

Steps to reproduce:

1. Open the attached world in 24w09a? or later

2. Observe that it takes a couple of seconds

3. Check your log for spam of:

   Tried to load invalid item: 'Item must not be minecraft:air'

4. Open the world again and observe it loading faster without log spam