[SPARK-17331][FOLLOWUP][ML][CORE] Avoid allocating 0-length arrays

## What changes were proposed in this pull request?

`Array[T]()` -> `Array.empty[T]` to avoid allocating 0-length arrays.
Use regex `find . -name '*.scala' | xargs -i bash -c 'egrep "Array\[[A-Za-z]+\]\(\)" -n {} && echo {}'` to find modification candidates.

cc srowen

## How was this patch tested?
existing tests

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #15564 from zhengruifeng/avoid_0_length_array.