G1 young gen sizer allows zero young gen with huge -XX:NewRatio

When passing only -XX:NewSize, the young gen sizer may return a minimum young gen size of 0 (unless young gen is huge).

There is no immediate impact with current code as other higher levels typically return more than zero regions (and even if zero regions for young gen were returned as "desired young gen length", there is no immediate impact other than a full gc).

It's merely an inconsistency with other results as other code prevents a voluntary 0-sized eden (which would result in a full gc) otherwise.