Crash when entity is leashed by non-LivingEntity

Summary

When an entity other than LivingEntity or leash knot (such as boats, minecarts, end crystals) uses a lead, the game crashes upon saving. (Relates to MC-272625)

Steps to Reproduce

1. Summon a Pig.

2. Summon an End Crystal.

3. Copy the UUID of the End Crystal with /data get entity @n[type=end_crystal] UUID.

4. Run /data merge entity @n[type=pig] {leash:{UUID:(insert UUID of end crystal).

5. The end crystal now leashes the pig.

6. Pause the game.