[SPARK-16561][MLLIB] fix multivarOnlineSummary min/max bug

## What changes were proposed in this pull request?

renaming var names to make code more clear:
nnz => weightSum
weightSum => totalWeightSum

and add a new member vector `nnz` (not `nnz` in previous code, which renamed to `weightSum`) to count each dimensions non-zero value number.
using `nnz` which I added above instead of `weightSum` when calculating min/max so that it fix several numerical error in some extreme case.

## How was this patch tested?

A new testcase added.

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #14216 from WeichenXu123/multivarOnlineSummary.