[MC-160464] New gamerules introduced in snapshot 19w36a default to false in worlds from older versions

Moderator Note

This bug was only fixed for worlds that were not opened in the affected snapshot. If you opened your world in 19w36a and are experiencing this problem, you need to execute following commands once (If cheats are disabled in your world; go to Open to LAN -> Allow Cheats: On -> Start LAN world):

/gamerule drowningDamage true
/gamerule fallDamage true
/gamerule fireDamage true
/gamerule doInsomnia true

I opened my existing world, and there was no fall damage, no drowning damage etc.
This is because the newly introduced gamerules get set default to false.

This should be fixed such that the gamerules default to true if they previously didn't exist.

Expected:
drowningDamage = true
fallDamage = true
fireDamage = true
doInsomnia = true

Actual:
drowningDamage = false
fallDamage = false
fireDamage = false
doInsomnia = false