stats: more conservative lower bound for downward-trending forecasts

Instead of 0 as a lower bound for predictions, use 1/3 times the lowest
prior observation. This will avoid predicting zero rows for a statistic
unless we really observed zero rows somewhat recently.

Fixes: #119967

Release note (bug fix): Statistics forecasts of zero rows can cause bad
plans. This commit changes forecasting to avoid predicting zero rows for
most downward-trending statistics.