The Bug:

It's important to note that wardens cannot deal damage to ender dragons using their sonic boom attack and this is an intentional game mechanic as stated in MC-250255, however, it seems rather strange that ender dragons still receive knockback from wardens' sonic boom attack. One would expect that if no damage is dealt, no knockback would either.

When the warden(s) attack the ender dragon, it takes knockback, which it shouldn't normally take from anything. I know this scenario would probably never even happen in a survival game, but still. 

Steps to Reproduce:

1. Summon some wardens and an ender dragon.
2. Allow the wardens to get angry and attempt to attack the ender dragon.
3. Wait for one of the wardens to perform a sonic boom attack on the ender dragon.
4. Take note of how the ender dragon doesn't take damage which is intentional (MC-250255) but still receives knockback from the attack which doesn't seem right.

Observed Behavior:

Ender dragons take knockback.

Expected Behavior:

Ender dragons would not take knockback.