I know there are some reports on this theme already, but this is about that you can't test for a player with a score of NULL which should be the initialized value for new created class or variable. (I hope this is correct I read this here: http://stackoverflow.com/questions/2707322/what-is-null-in-java).
So then my question is, as MC-50601 "works as inteded". How can you then testfor a player with a score of null when

/testfor @p[score_Test=null]

doesn't work

I don't know if this is helpful, but wouldn't it be possible to either add/fix it that you can test for players with score of null or when you test for a score of 0, the game also looks for players with score of null

Maybe another argument for you that this is a bug: You can test for players without a team (/testfor @p[team=]), altough they probably have as team also null