[SPARK-18036][ML][MLLIB] Fixing decision trees handling edge cases

## What changes were proposed in this pull request?

Decision trees/GBT/RF do not handle edge cases such as constant features or empty features.
In the case of constant features we choose any arbitrary split instead of failing with a cryptic error message.
In the case of empty features we fail with a better error message stating:
DecisionTree requires number of features > 0, but was given an empty features vector
Instead of the cryptic error message:
java.lang.UnsupportedOperationException: empty.max

## How was this patch tested?

Unit tests are added in the patch for:
DecisionTreeRegressor
GBTRegressor
Random Forest Regressor

Author: Ilya Matiach <ilmat@microsoft.com>

Closes #16377 from imatiach-msft/ilmat/fix-decision-tree.