'/execute (if|unless) score' cannot compare scores that are not set

The bug

With the new /execute command syntax, the sub-commands if score and unless score should be able to compare scores to players that don't have their scores set. This does not work, however; an error is returned about how there is no tracked player on the scoreboard, which does not line up with behavior in Java Edition.

In Java Edition, /execute (if|unless) score can compare scores that aren't set and is often used for initializing scores. Comparing an undefined score to a value always returns false.

Steps to reproduce:

• Create a new scoreboard objective and set some score on it. I'll be using a fake player named "one".

  /scoreboard objectives add foo dummy
  /scoreboard players set one foo 1
  

• Compare your currently undefined score to the "one" score.

  /execute if score @s foo = one foo
  

→ "There is no tracked player '<player>' on the scoreboard; Execute subcommand if score test failed."