Shulkers don't have their innate armor attribute modifier after spawning as closed until they open once

Shulkers have an attribute modifier that increases their minecraft:armor attribute by 20 when they are closed, but not immediately after they spawn as closed; they have to open once and then close again to be given the modifier.

Note that this is different from MC-272887, since shulkers with NoAI are never given the modifier, even if they are forcibly opened and closed using commands.

How to reproduce:

1. Summon a shulker.
2. Before the shulker opens up for the first time, execute the following command:

   /attribute @n[type=minecraft:shulker] minecraft:armor modifier value get minecraft:covered

   → The attribute modifier doesn't exist.

Expected result:

The shulker is closed, so it would have 20 innate armor points.

Observed result:

The shulker doesn't have any armor points despite being closed.