• Run : /summon minecraft:dolphin ~ ~ ~ {}

→ If out of water, the dolphin will instantly start to take damage.

→ If under water, the dolphin will instantly start to take damage.

For now, everything is normal (takes damage out of water because Moistness=0, and under water because Air=0, since tags were empty in the command)

• Run : /summon minecraft:dolphin ~ ~ ~

→ If out of water, the dolphin will (still) instantly start to take damage.

→ If under water, the dolphin will not (no longer) instantly start to take damage.

► When leaving completely empty (meaning "not even the brackets") the NBT tag argument, the entity should spawn with its usual tags, like when using a spawn egg. This is not what happens here, for the dolphin's "Moistness" tag. Under water, the dolphin instantly resets its Moistness tag (... since under water), so they avoid damages. But of course, they still spawn with Moistness=0.