Players do not have a default score of 0 for an old or new objective.

Players do not have a default score of 0 for an old or new objective, it must be given to them first. Therefor there is no way of detecting a player with no score at all, as a default score.

So for example if you had an objective named "code", this command doesn't work
"/scoreboard players test @p code 0 0"

Unless this has been excited before hand
"/scoreboard players add @p code 0"