## What changes were proposed in this pull request?

A nonsensical split is produced from method `findSplitsForContinuousFeature` for decision trees. This PR removes the superfluous split and updates unit tests accordingly. Additionally, an assertion to check that the number of found splits is `> 0` is removed, and instead features with zero possible splits are ignored.

## How was this patch tested?

A unit test was added to check that finding splits for a constant feature produces an empty array.

Author: sethah <seth.hendrickson16@gmail.com>

Closes #12374 from sethah/SPARK-14610.