Modifying the file /datapacks/MyDatapack/data/minecraft/tags/blocks/valid_spawn.json to contain less then 2 values will crash the game (Crash Report attached)
 

(the game crashes once a mob is tryed to be spawned.)

Depending on the Crash report i woud assume the game is trying to choose a random block of that list using the Random class. I guess the value "2" is hardcoded in such a way that the bound will be below 0 if the amount of blocks in the list isnt 2 or higher.
Depending on this, i also assume any block above the second element will be ignore, but i had to investigate further for that.

Edit 1: It appears that any edit to the file will crash the game